I think in the long run of this game it will not matter, but at the moment Nvidia cards are better just because Schema probably makes the game on a machine with an Nvidia card. Just don't get a GTX970, it runs starmade well, but I had performance dips pretty high when it comes to massive objects on maxed out settings before I returned mine. Buying a good GPU is something you should expect to last a long time, the 970 is gimped by design and will not have anything in the way of longevity especially if you want to play Starmade higher than 1080p. If you value your dollar/whatever currency you use, either shell out for a 980, wait for AMD's new cards in a few months, or wait more months for the inevitable GTX980 ti or Nvidia's pascal.
If you're strapped for cash, go with the higher end of the 700 series or get an R9 290.