First, since 2.4 GHz is the latest and greatest, much of the new innovations in cordless products are appearing on 2.4 GHz products only. In particular, the new crop of multi-handset cordless systems -- which are better than ever -- are all 2.4 GHz products. So, if you want to have one base and several cordless handsets, with the ability to intercom each other, transfer calls, and other big-system features, then the choice is easy: you must buy 2.4 GHz.
Our local users, many of whom have installed these products in offices or large houses, find mixed results. In some cases, 900 MHz has worked better (meaning less fade, dropout, static) than 2.4 GHz. But in other cases, 2.4 GHz has been the clear winner.
There is no definite answer we can give, because all wireless products are sensitive to environmental issues such as wall construction (wood, steel, masonry), presence of other electronic devices (although both 900 MHz and 2.4 GHz phones seem much less affected by interference than the early-model cordless phones were), and location of the base.
If you are looking for absolutely the longest-range cordless money can buy, check out our line of Engenius wireless phones.