Both have their pros and cons. Recently I've liked ATI more.
I like nVIDIA because they run cooler, use less energy, are relatively cheap, and for those who must have SLI, it's an already proven and mature system of having dual-cards. However, I like ATI because of their smart moves in architecture improvements like the R520 and R580's 512-bit Memory Ring Bus which is very unique and has A LOT of bandwidth, not to mention the initial move into the 90nm process. They also have better AA and AF than nVIDIA does, not to mention AVIVO, which is similar to nVIDIA's PureVideo, but you don't need to pay $30 extra to get the PureVideo encoder in order to experience High-Def videos. AVIVO is free out of the box.
So I guess it really depends.