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I could spend three hours talking about the many advantages of 5 GHz (802.11a) over 2.4 GHz (802.11g).

However, I highly encourage anyone to go with 5 GHz if they plan on deploying VoIP. The timely delivery of VoIP packets is extremely susceptible to Layer 2 retransmissions. The 2.4 GHz environment is a high retransmission environment and nowhere near as friendly to VoIP.

Very quickly, below are several reasons that 5 GHz is preferable to 2.4 GHz, especially when voice is deployed:

A) 5 GHz UNII bands have no where near the non-802.11 sources of RF interference that exist at 5 GHz

B) Due to the availability of many more channels at 5 GHz, both co-channel and adjacent channel interference can be completely avoided with proper channel reuse design at 5 GHz. No so at 2.4 GHz.

C) Interference from 5 GHz WLANs from neighboring businesses is usually not an issue.

There are disadvantages to 5 GHz and VoIP. There are issues with roaming with the VoIP phones in UNII-2 and UNII-2E because of the radar detection requirements. But that is another post for another day.