I've read that 5GHz has a problem with physical contention compared to 2.4GHz. That is to say with a concrete, stone, or double brick wall in the way, 2.4GHz will still get through door ways and as long as positioning of the access point is OK, you'll still get some signal to/from wireless clients. 5GHz is not as forgiving.
Apparently 5GHz has a further broadcast radius in an open environment, as in without physical contention. So this can make the choice on configuration confusing, and especially when considering purchasing an 802.11n access point and client devices, and investigating physical planning and locating. Trial and error (without complex site survey equipment) is probably the best approach. If you're getting the kind of performance you want on 802.11g, I'd probably advise to stick with it.
For reference, my Time Capsule is in mixed mode. I've seen 110Mb on the MacBook my better half uses. She thinks the performance is great, and its user perception that's important. My iMac is on 1Gb Ethernet, and my PS3 is on Ethernet as well (this helps to keep Ping rates at a minimum). My Wii uses 802.11g and seems fine, even though the access point is two rooms away.
The one big draw back with Apple Airport is that there are no external 'F' type connectors to attach hi-gain or directional aerials. If you find your signal strength drops or you have black spots or no coverage areas in your house, the best bet is to choose an access point where you can attach hi-gain aerials, and MIMO access points and routers can help further by allowing you to attach 2 or more.
Hope this helps...