I looked at Catalina AL 100's a while back, but they're huge (tall, as well as 8'' diameter), and they have an operating pressure of 3300 as opposed to 3000 for an AL 80. I don't think the 8'' diameter will fit in the tank racks of commercial boats.
Yeah, those things are huge and heavy for the amount of air you can get into one. I wouldn't recommend them for singles or doubles.
If I get AL 80's, they might be able to get doubled up later.
I guess this depends on the type of diving you're doing. If you're not going deep (past recreational limits) they're fine. Otherwise I'd suggest something you can get more back gas into.
Steels sounds nice, but WAY too expensive for a college student like myself.
You can find good used tanks on eBay, Craigslist, diving boards, etc. for reasonable prices. Look for those with fairly recent manufacture dates and current hydros. You might have to have them inspected ($5), O2 cleaned ($40), and/or hydro tested ($40). Just include those costs when you're looking at a used tank to buy.
I want all of my diving to be on Nitrox from now on, if possible.
Al and steel require O2 cleaning for nitrox fills...as do your regs. The nitrox piece isn't nearly as much of a factor as the others that have been discussed in this thread IMHO.
What do I recommend? All my back gas tanks are steel. I like the fact that I can get a lot of air in them and how they help me trim out. I have LP85s for my doubles...about 225cf with a 3500psi fill. They're relatively small and light. My two favorite singles are a LP85 and a HP120. For a single tank the HP120 is pretty nice. Not too heavy or long and holds just over 122 cf of gas when full (3500 psi). Compared to an Al 80 the tank is less than 2" taller, about 8 lbs heavier, and has the same diameter. I know a couple of guys who have tried my HP120 and have then gone out and bought one for themselves.
I lot of folks like either LP85s or HP100s because of the capacity and size:weight ratio. Here's a link to a page that has specs on many different aluminum and steel tanks:
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan
Be sure to consider the differences between the weight of the tank empty as opposed to full as that may affect how much weight you'll have to add to your belt. Personally, I don't add any weight to my belt with my steel tanks unless I'm diving a thick wetsuit or a drysuit. I do have to add weight with aluminum tanks.
Good luck!!