@BJ You're really in the sweet spot for usability/durability of GM trucks in that date range. Here are the advantages/disadvantages as I know them, with the disclaimer that I have not owned all of them, but I have experience with all of them.
GMT-400:
'95 is the first year of the revised interior, which I prefer. It's a much more modern/usable layout. Purists prefer the older interior which is perhaps more in "style" to the truck, but it's functionally worse in every way. '95 & '96 will have a driver's side airbag, '97+ it sprouts a passenger side airbag as well.
'95 is the last year of the TBI engines. My dad had a '95 with a 350, and I believe it was rated at 200 hp. In '96 the Vortec V8 dropped, and horsepower leapt up to 260! '96 was also the first year that the 3rd door was available on extended cabs.
Overall, the 400 platform is a very excellent and very stylish platform. It's biggest weaknesses as I've experienced them are the front end parts wear out much quicker than the 800 platform, and boy are they a DEVIL to drive when they are cashed out. BUT, as covered, parts are cheap and plentiful. The other big weakness with the platform are mushy brakes. Even at their best, they suck. As I understand, an upgrade to a master cylinder out of a GMT-800 truck makes a world of difference here. I believe @blacktruck18 performed this mod on his old GMT-400.
The GMT-800 platform, '99-2006 (2007 classic), is in my opinion the greatest truck platform ever built. I've had a 2006 GMC Sierra since new, and it has never stranded me, nor stuck me with a repair bill over $600 (rust repair not withstanding). Still very stylish, still the right size. This is the platform that the LS based 4.8/5.3/6.0 liter engines debuted on. These engines routinely see 300,000+ miles with only the most basic of maintenance. The front ends are much tougher than the 400, and the brakes are a significant upgrade was well. The 800 platform debuted with standard 4-wheel disc brakes, but by the time my '06 was built, base models had been decontented to include rear drum brakes. Even still, they're a lot better than the brakes you'll find on a 400 series truck.
If you're in an area with rust, the rocker panels will dissolve from the inside out. Especially on an extended cab truck. Aftermarket replacements are available. I had mine redone a couple years ago. The brake lines are prone to rust as well, but by now these should have already been replaced on a well maintained truck. GM offers a very cheap kit of replacement powder coated lines, so look and see if the brake hardlines are now black, and you'll know that this has already been done. If you instead see a patchwork of bullshit, find a different truck.
Also look at the transmission cooler lines, I just had those rust out on mine too.
1/2 ton 800s are at the bottom of the depreciation curve. If I had the space and money, I'd be snapping up every clean one I could find right now.