Don't bother installing a 2x2 card if you aren't going to connect it to a well-designed, well-placed second antenna.
I've seen 2x2 cards/drivers choke when one of their antennas is disconnected. It seemed as if the card wasted time trying to do 2x2 that would never work, instead of just sticking to 1x1 operation.
Besides, a 1x1 AC card can reach PHY rates up to 433Mbps, whereas a 2x2 N card (such as the 7260AN you're looking at) would max out at 300Mbps even if both radio chains were hooked up to good antennas. If only giving one radio chain an antenna, that card will max out at 150Mbps. So go with the 433Mbps card instead of the 150Mbps card.