5.x GHz signals penetrate walls less well than 2.4 GHz signals. 900 MHz is better than both, which is why I'm miffed all the wireless phones moved up to 5.8 GHz in this pointless 'GHz race.' The advantages of moving to 5.x for computer networks is that you can use wider channels to send more data (which isn't necessary for phones - they should have stayed where they were), and that you have less interference, as others mentioned.
If you're worried about range, try putting your new base station to 2.4 GHz, and see if it improves. If you're seeing (and making use of) real-world speed improvements at 5, you're probably better of staying there.
Basically, there ain't no such thing as a free lunch. There's a reason the lower spectrum bands are so coveted, and the cell phone providers are chomping at the bit to kick TV stations out of the UHF band. In general, signals at a lower frequency travel further, but you're not going to be able to get a 40 MHz wide signal in the kHz band, obviously.