I've worked in a house that offered PADI and UTD.
When a customer walked in asking for a diving certificate, I'd tell them that we had two options - both were valid to 18m, world-wide and for life. One cost approx. $500, the other $600.
Everyone would ask "so what's the difference?".
I'd say the $600 would focus more on diver technique using skills that are universal through any type of diving, and using educational tools such as video debriefing.
That's it.
PADI - respect to them for being the first to "make it" in scuba - had completely different prerequisites for establishing a "how to" teach diving back in the 1960´s, and unfortunately, the legal paradigm is that if an agency changes practices, it makes them (unreasonably) liable in terms of prior cases. I think this is a problem, and a great injustice to agencies and divers alike.
Regardless, PADI's training paradigm is not designed for technical diving - i do mean everything from gas/dive planning to skills and even equipment. It makes perfect sense that DSAT takes over when crosssing into that domain, as PADI simply don't focus on that area of the scuba business.
And guys - that's fair enough. A lot of divers do NOT want to go anywhere near tech training, and are perfectly happy with a cert that allows them to go on an annual dive trip with a guide. No problem.
The GAP between Rec and Tech
Personally, I started my tech diving within the framework of DSAT.
My take on it is that is an immediate issue with the first level of DSAT-training, in that their first course is an actual tech course where students will learn to carry out entry-level technical diving, including of course a deco obligation.
My personal experience was that a lot of students on that course, spend a disproportionate amount of that course working on basic diving technique (most or all of which has relevance in recreational diving as well as technical diving).
A Danish dive centre had their in-house Course Director develop an "Advanced Dive Skills Specialty", which is PADI-approved, specifically to address this issue.
Divers who have been on a UTD Essentials, GUE Fundies or ISE Basics, will recognize the gap that is being addressed on that specialty course.
I feel it's an issue that there is a formal and cultural "gap" across Recreational and Technical diving, in spite of the fact that - in my view - the foundation of Technical training IS Recreational diving.
I can't speak for GUE (I am a UTD-instructor) but I find that it makes a world of sense to perceive diving as one single area within which the diver/student can develop towards whichever branch of diving, using the same foundation and skills. This makes for a lean, economic and linear development as divers progress.
I think the value of this is hard to overemphasise.
It's the instructor, not the agency
You'll hear this often online, "it's the instructor, not the agency". And it's absolutely true - for some agencies.
There is a vast difference across an agency which aims to give it's instructor core as much freedom to pursue the highest echelon of diver training on parameters that they perceive to be of pinnacle importance, and an agency that aims to secure a (high) minimal capacity.
That is, the difference is fundamentally significant across how those two fictive agencies approach matters, for instance in terms of Quality Assurance and general training.
Granted, in a world where it's easy and transparent for customers (divers) to evaluate one product (course/instructor) against another, I think it makes sense to give instructors as much freedom as possible, including at the expense of formal standardization. Consider the hotel sector - of course, there are vary basic, fundamental things that all hotels must adhere to, for instance fire code or tax laws; but other than that, it's relatively easy for customers to get a grasp on which hotel to go for:
Parameters are reasonably straightforward; cost, location, quality.
Quality is easy to figure out, because there is a 5-star system that pretty much everyone uses, and gives a very good idea of the quality level, at least ballpark. And prior customers' recommendations give a generally reasonable idea of their experience with the product (hotel).
Now try the same in scuba:
Sure, cost is easy to evaluate. So is location.
But quality? There is NO universal system that customers can go by.
Pretty much every recommendation from prior customers says the instructor is "great!", most likely because divers rarely have a large number of instructors to compare across, and diving is by default most often pretty awesome.
Completely different ballgame.
Literally the only guideline that there is, are RSTC guidelines (the contemporary relevance of which - or lack thereof - I think deserves its own thread).
My view on it is that PADI do not aim to secure a high level of diver capacity relevant to technical diving.
When jumpiing into Technical training with their tech-umbrella (DSAT), you're starting over.
And even then, you have no way to gauge where the bar is, or if you're getting training of the highest order, or not.
There are stellar instructors with DSAT and PADI out there, don't get me wrong.
But it's hard for you to figure out who they are, that's my point.
With agencies such as UTD (and, from my understanding, GUE and ISE), the view on training is fundamentally different.
The anchor is placed in the heavy end of the spectrum, and based 100% on real life experience with that sort of diving - by divers, for divers - and then scaled down from there.
On the other hand, in my experience, the price is often higher on those sort of courses, and availability is almost universally lower.
For me, personally, I am happy with the development I've made in diving, where I am now, and comfortable with my path forward - as well as thankful to DSAT/PADI for helping me get started.
I hope the above serves to help make a decision, or at least offer a bit of inspiration if you are looking for your first or next course.
In either case, have fun diving, and best regards.