To answer your question I must first identify some possible prejudices and some ignorances. I am an IANTD, TDI, and PADI instructor. So I may show some prejudice towards what I know. My ignorance is; all I know about GUE is stuff I read in areas like this.
I have read many of the postings relating to your question with interest and feel that I should jump in on a few areas. First when I refer to TDI you can inset IANTD [with some smudging of course disc, etc]
First the number one thing you need to decide is what direction your diving is taking you in. You have decided to take technical courses but for what reason. Deep diving, wreck diving, cave, etc. this will help you decide what courses you need and possible what agency to receive those courses from.
As for TDI separating out the different levels and areas of training this was based on consumer demand. Believe it or not there are divers who do deep air dives specifically east coast wreck divers. TDI does not say that this is the only way to dive but they will offer you the training if you require it. With that in mind they have separated out the Advanced Nitrox form the Decompression Procedures. [Not many people take just the Advanced Nitrox]
90% of the time these two courses are taught together but there are a select few and most of those certifications come from the north east coast are for deco procedures and deep air.
TDI has set up their courses so that several of them can be combined together [at a reduced cost] so that those people who have a need can pursue it. I charge $700.00 Canadian dollars for the combined Advanced Nitrox and Decompression Procedures course. Some people are using these course to do long dives at 100 130 feet were we have several great wrecks, therefore they do not need Trimix but still want to spend 60 minutes at 100 feet. With the above courses they can accelerate their decompression time considerably
Unlike some other agencies that only teach their method or nothing. Which may or may not meet all divers needs or work in all situations. TDI is trying to reach a market that is much more diverse in divers needs.
I have trouble with the posting that state that anything bellow 100 feet requires Trimix. I am a Trimix diver and have about 100 dives below 300 feet on Trimix and about 100 dives bellow 200 feet on air. Remember Trimix is relatively new to the diving world back in the early 80s in Hawaii we never even heard of it in the recreational diving area [the word tech diving had not been born yet] so like Brett Gilliam we to were doing deep air dives because it was what was available, and due to working as a diver and doing repetitive dives on air sometimes four a day to 100 feet we were able to handle the narcosis associated with deep air [along with the other things that were not open information at that time]
So as technology changed most deep air divers move happily to Trimix it just meant that they could go to those wrecks for longer times.
I currently keep my END above 150 for deep dives or challenging dives.
As for GUE they do have a good reputation for teaching cave divers, and like I said I know little about them other than the consider themselves the know it all of teaching agencies for tech diving.
As for selecting an instructor this is the most important step more so than the agency. Like I said earlier once you have decided what kind of diving you are going to be doing then find a instructor who is doing the same kind of diving. An important thing I tell everybody I come into contact with is stay away from instructors that theyre only diving during a course. Some instructors get so busy teaching they get away from just diving. This tends to get them out of the loop of the real world.
Once you have found this instructor ask him if you can meet him at the site when he is teaching another group of divers listen in to his teaching mannerism and see that it matches up. Find out if he is only teaching classes as a loss leader to sell dive gear. Does he get his point across to his students, do his students seem prepared to get in the water and do the diving he is talking about.
I personally dont think any of the recognized training agencies are bad, there are only bad instructors, and you will only get out of it what the instructor and you put into it.
If you are thinking about cave diving [I think one posting referred to it] go ask those people who they recommend, it is a pretty small community and you will get a lot of opinions but you will be able to come up with a common denominator. Go visit the dive site and watch you will see who is doing real tech diving; they will have an instructor to recommend.
As for gear 40cu ft or what ever else. When you get ready to get into tech diving be prepared to spend huge money, select your gear then find the best price on it. Dont do it the other way around. As for size of stage bottles that comes right back to what kind of diving you are doing. And what kind of problems you are allowing for, longer than expected deco, or just long deco, loss of gas, silt out, etc, etc.
My personal gear is as follows for deep wrecks [deeper than 250 feet]
OMS dual bladder 100# lift
Aluminum back plate
Double OMS 112s with OMS isolation valves
1 Small light attached to my shoulder harness for reading gauges
Two lift bags
Two reels small and large
Spare mask, spare light, Med scissors, leatherman, and knife
OMS VR3 computer
Two OMS bottom timers
Wrist slate with dive profile written on it
Thermal protection is an Oceaner 7 ml neoprene Drysuit with pee valve, gloves and hood
Argon bottle
Side mounted tanks [stage bottles] this will get you taking]
Left side EAN 36 in aluminum 80 cu ft
Right side EAN 80 in aluminum 80 cu ft
I have even been know to bring an extra 40 cu ft on my right side if the dive requires it
[As I push to 400 ft I am looking at slinging Faber 95s on the side]
Main light is an OMS HID canister light
Regulators; I am a long hose breather, with my alternate around my neck
I use the Poseidon Odin regs with a 7 long hose on my dbls
Atomic Titanium regs on my travel gas
OMS oxygen cleaned reg on my high O2 tanks
My tank selection is based on my deco times 2 hours plus. Yesterday I had a 90-minute deco time and it seemed so short. Not being an air pig, but this could also be a factor.
My normal sac rate is .5 cu ft / minute, but for instance the other day I bumped into [literally] a dead scuba diver at 340 feet it turned out that he had been there for six years, I ran across him at the end of my dive, needles to say my sac rate increased dramatically and I silted the place out a bit. As a result my bottom time was extended by about four minutes. However because of my tank selection I could extend my deco times, and breathe heavily for a few minutes until I resolved the situation. I tied my spare real to his tank valve and found the exit, then tied it off there. [However I digress]
Your last question decide what gear you are going to buy then find the best deal you can weighing out the warranty, and maintenance issues against money saved.
Keep in mind my opinions are my own and do not represent the opinions of any agency I am associated with
=-)
. I should be getting some work done 
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