a - 55Mbps
b - 11Mbps
All depends on range and if the access point / router supports the newer 55Mbps speed. Price will be much higher for the 55Mbps as well.
I just bought the Linksys Wireless Router / NIC.
The NIC I got was version 2.5 which was the latest until I just noticed they have a version 3 out. I have heard from a few people who upgraded from version 1 to 2.5 that it was a good improvement. Only problem was that the model number stayed the same, so when I was ordering, I did not know which I was going to receive. I ordered both from Dell with a $25 off $200 and a 10% off coupon (
http://www.hotdealsclub.com had them listed) .
I ended up getting the newer 2.5 NIC, and the older Version 1 Router. Both work as well as I need them to. I only live in an apt, but I receive excellent signal quality from anywhere in the Apt. This of course will vary home to home depending on how many walls are between you and the router, etc.