Gigabyte and EVGA are both great brands that have proven themselves in the video card segment. I'm sure you'd be just as happy performance wise with either card.
In regards to the warranty, it's true certain video cards from EVGA do not have a lifetime warranty. I would highly suggest going to EVGA's video card product page and inspecting each one's warranty individually before purchasing.http://www.evga.com/products/prodlist.asp
Personally, I had a video card burn out in the past from EVGA (Not their fault, it was a 9800gx2 which was notorious for burning out due to heat issues) and since it was covered under a lifetime warranty, they sent me a shiny new GTX 285, which was a 30% performance increase for no cost and less heat.