802.11a vs. 802.11b
As you probably know, 802.11a and 802.11b each define a different physical layer. 802.11b radios transmit at 2.4 GHz and send data up to 11 Mbps using direct sequence spread spectrum modulation; whereas, 802.11a radios transmit at 5 GHz and send data up to 54 Mbps using OFDM (Orthogonal Frequency Division Multiplexing).
Of course the superior performance of 802.11a offers excellent support for bandwidth hungry applications, but the higher operating frequency equates to relatively shorter range. I've seen demonstrations of 802.11a radios delivering 54 Mbps with distances of about 60 feet, which is far less than the 300 feet or so that you'll have with 802.11b systems. As compared to 802.11b, you'll need a much larger number of 802.11a access points to cover a facility, especially large ones.
The different radio frequency and modulation types of 802.11a and 802.11b causes them to not interoperate. For example, an end user equipped with an 802.11a radio card will not be able to connect with an 802.11b access point. The 802.11 standard offers no provisions for interoperability between the different physical layers.