I go with what is best at the time for my budget. Right now that's ATI, my previous 2 builds were nvidia, and the one before that was ATI.
I could care less about company loyalty, even though it may looks like I'm playing for ATI that's only because, I think their cards this gen are better and the Nvidia Fanboys drown you out unless you push hard.
I have 2 4850s in crossfire, which for all intents and purposes is the same thing as a 4850x2, and I can say, that the performance is pretty nice. This is my first multi-gpu setup and I am very happy with it. Scaling in games is good and I can easily blow through all the major titles at my resolution (1680x1050).
Furthermore, recent anandtech benchmarks have sown that with the recent drive updates, ATI cards are performing much better over the Nvidia ones. The 4870 1gig, used to not beat the GTX260, now it beats it in 11 out of 12 tests. that's at low and high resolution. the benchmarks in question take the full spectrum of cards out now so it will take some time to comb over, but from the looks of it, we will have to change our minds on a lot more cards. What helps the 4870 also helps the 4850, and 4850x2 and 4870x2.