It really depends on your needs. Where Nvidia often make the truly top end and high tech graphics cards, they come at a premium price and cost a lot more than an AMD card at a similar level. AMD cards however, nowadays, produce a lot more heat and use a lot more power than even the most powerful of Nvidia cards.
There isn't really a comparison there, it's just a general summary.
If you're looking for a top end card, I would recommend a GTX 980, R9 295X2, or GTX Titan X.
If you're looking for a high end card, I recommend a GTX 970 or an R9 290/290X
If you want a mid range card, I recommend an R9 280 or a GTX 960
If you want an entry level gaming card, I recommend the R7 270
If you really want a budget card, I recommend the R7 260X or the GTX 750.
Woody