This is a metropolitan legend that maybe was true many years ago, but not now. The problem with these things is that people only remember bad things and bring them on in the future. Many years ago Ati cards had many problems with drivers and so they had the reputation of being worser than Nvidia cards. However things have changed today and Catalyst drivers are on par with Forceware ones (the only big difference is for what concerns SLI/Crossfire profiles, but it's gonna change soon). Many still believe that Ati cards are inferior apart from drivers and card-to-card benchmarks because they don't have the same Physics engine as Nvidia nor they can use some software made specifically for them for video making (or they are not so diffused).
The truth of the matter is that it depends primarly on what games you want to play: many games play almost the same (naturally depending on type of card) with Nvidia and Ati, but there are some titles that prefer an architecture. Dead Space for example is very bad for Ati cards, while Conan is worser for Nvidia. Some titles then uses integrated GPU phychics made by Nvidia and can have some problems with Ati cards, but in the majority of cases the challenge is more based on hardware than these sort of things.
Ati cards cost less and perform almost on par with Nvidia cards (at last in the majority of cases, there are exceptions of course; for example the higher end single-gpu Nvidia card is much better than the same by Ati, but the prince change is steep). The drivers are good for both brandings. It depends only if you want to pay more for some things that Nvidia has and Ati doesn't, if the increase in performance in some cases or if the games you like to play on doesn't use Ati so well and if these things suffice the increased price. Some people think that paying more for it is justified, some thinks not. I suggest you to make your own opinion based on facts (i.e. benchmarks of titles you know you want to play) instead of relying in legends or earsay.
What I can tell you for sure is that many hardware sites (and also TH) have no problems on suggesting readers to buy Ati cards for systems, and on the contrary many "systems for the money" are suggested with Ati cards. Do you believe more what your friends says relying on earsay or what experts think on the matter?