The absorption of the signal does vary with the frequency. It would take a boatload or equipment to measure this but you should...
Go with what works best.
Just got the NETGEAR - RangeMax Dual-Band Wireless-N and set it up (no issues setting it up, in fact surprisingly easy)and it is set to dual band (2.4 & 5GHz).
House has concrete walls (I know this can mess with the signal)
I get an excellent signal with the 2.4GHz wifi (in the room where I need the laptop working), but the 5GHz signal is not that great.