I'll forget for a minute that ATI never made anything called a 6800 Pro (or anything even close to that), and that nVidia hasn't yet released their newfangled 8800 GTX (and is nowhere near anything called a 9500)... Sorry, couldn't resist.
(I'm sure you meant an nVidia 5900 and I'm not sure where you were going with the ATI, but maybe a 8500?)
EDIT: Actually, maybe you meant an ATI 9500 and an nVidia 6800 which are real cards. Comparing those two cards would certainly lean in nVidia's direction though, as they are from completely different classes of cards! The 6800 was a top-of-the-line nVidia card which debuted for $450. The 9500 was based on the old 8500 core and was targetted at low-mid range and debuted at around $150-$200 if I remember correctly (the 9500 was also a terrible and rushed GPU which was swiftly replaced by the 9600 which was pretty good).
To some degree the decision between ATI and nVidia is a personal choice issue. I find exactly the opposite of what you found in your (admittedly cursory) flirtation with ATI.
I find nVidia's drivers horribly buggy and annoying to use (and I do currently own a 7800 GTX so this is based on experience). ATI's are really no huge joy either, but at least I don't have to remember a huge list of "ands, ifs, and buts" about what works and what doesn't. nVidia's cards are very good for certain applications (OpenGL for example), and their drivers are good enough if all you want to do is set it up like a typical PC with one or two monitors. Otherwise, I personally find ATI's Catalyst drivers vastly superior. I also really appreciate their release cycle (a new driver set every month, clearly named). nVidia releases new drivers when they feel like it, and more often than not, the performance goes backwards not forwards.
The only reason I bring this up at all, is a lot of the commentary I see about the issue (people complaining about ATI's driver quality) is based on experience they had in the Radeon 7000 and 8000 series era many years ago (2001 and 2002). I'm the first to admit it! ATI's drivers then were horrible. They were buggy, they crashed, features didn't work, they had quality issues, and on and on... But, it's important to give credit where credit is due! They responded to the overwhelming criticism of the poor quality with the Catalyst driver program, with a vow to fix the issues, and to release new versions once per month, and to support them well. And they did.
I'd say basically the exact opposite story about nVidia. They've basically been heading downhill driver wise since the nForce2 era. Their nForce4 chipset is still plagued with driver issues, their GPU chipset drivers have terrible image quality problems (what nVidia called Anti-Aliasing and what ATI calls Anti-Aliasing are not the same thing at all, and nVidia has quality problems in many current games which make running the cards at their full potential unworkable), and they seem to turn off features in new drivers more often than they introduce new ones (nForce LAN acceleration anyone?).
I think the problem really comes down to one of resting on your laurels. Back in 2002, everyone laughed at nVidia's hardware (the 5x series GeForce cards got creamed by ATI's 9700 in basically every performance category) but praised their drivers. I think that was mainly because they were searching for something to praise about an otherwise failed product, but still, compared to what ATI was offering at the time, anything was good. nVidia set about fixing their hardware problems (which they've certainly done), but at the expense of ignoring their drivers.
Again... The decision is certainly personal. Some people just don't like the way ATI's software is set up! Some people (me) also just don't like the way nVidia's is.
I would dispute the "great bits of kit, if you can keep 'em running..." part of your statement. My ATI cards have been, for the most part, completely trouble free. I've had equal or greater problems with my nVidia cards over time (and in at least a few cases, there was never a solution to the nVidia problems -- like the horrible flickering when entering and exiting full screen mode on my 7800 GTX).