I guess fidelity of detail and aesthetics is my thing. A good example of a comparison of two like-kits would be the Hasegawa and Tamiya P-51D's.
They both have their pros and cons, but for me the pros of the Hasegawa kit outweigh those of the Tamiya kit; even though the Tamiya kit is, overall, a much easier build.
I feel that the Hasegawa kit, built SOB, has better detail than Tamiya's, especially in the areas of the cockpit, gear legs, gear wells and gear doors. (even though the wells are too shallow) And the canopy and windscreen on the Tamiya kit are, in my opinion, a damn shame. They're entirely too thick and although the idea of the two piece hood is good in theory, in practice it's a joke and looks horrible. This alone ruins the kit for me. If only I got along better with vacuform canopies...
The main issue I have with the Hasegawa kit is the fact that the flaps cannot be dropped without a bit of cutting and reconstruction. But that's not a problem for me, as will become apparent a bit farther down this post. (If anyone gets that far...)
For me, unlike many folks, price is a non-issue. I do tend to grab stuff inexpensively whenever I can, but I have no problem in picking up a kit for retail if it's something I want to build.
As to buildability; ease of assembly, fit problems and what not, I'm really not daunted by a whole lot out there. I've been building for twenty years and I know my way around an aircraft model kit and, to a lesser degree, a military vehicle. So that's not much of an issue for me either, and I'll explain why...
When I first got into the hobby, the hot-sh** kits were Monogram. Recessed panel lines were still the exception, and the so-called norm of the shake-'n-bake kit was a long way off yet.
Don't get me wrong, there are no perfect kits, even today. But those which were available over the course of my first ten years of modeling are what many of today's newer modelers scowl at as being obsolete, ill-fitting and, for some, simply not worth the time. That's the kind of stuff I 'grew up' with, and 'matured' with as a modeler. You had to really work on most of those kits to get a good result. Things didn't always line up as well as they do on some of today's kits.
Nowadays, kits seem to be judged largely by whether or not you're gonna need to use a tiny bit of filler here and there.
OH MY GOODNESS! YOU HAD TO USE FILLER?!?!? Back in the day, as the kids say, (HEY! I was a poet and wasn't aware of the fact...) we
took it for granted that we were gonna need some filler on almost every kit we built.
We also didn't have the proliferation of brass and resin fixins which you see today, either. White metal was the thing back then, and the selection was pretty limited. If you wanted to have a superdetailed cockpit, you had to scratch it.
So when I hear or read some folks today complaining about kits from companies 'X' and 'Y' because they didn't fit all that great or the detail was sparse, I just laugh to myself and am glad that I grew up as more of a model builder and not simply a model assembler.
I've become lazy, though, and will often times ditch an older kit if a new one becomes available. In some cases, as with the Monogram and Tamiya Do 335's, it's a no brainer. With others, like DML and Tamiya Fw 190's, it's a tough call. So, in the end, I wound up keeping most of my DML/Dragon and Trimaster 190's because they're good kits. Still grabbed a few Tamiya 190's though.
So, after all that, when it all boils down, my answer, in a nutshell, would have to be:
I'll build just about any kit so long as it's something I want to build.
And so long as it's 48th. Although I'm slowly making an allowance for braille scale...
Fade to Black...