The drawbacks of the original Xbox adapter are:
1) Size. Obviously, it's not massive, but it will take up more space than the 360 antenna.
2) Power. The Xbox adapter needs external power (ie, plugged into the wall), while the 360 adapter is USB-powered.
3) Security. The Xbox adapter only works with WEP, which is the least secure encryption standard that your router uses (well, other than no encryption, at all). It's not really a massive deal, because chances are, no one really cares about your home network.
Personally, if you can save a lot of money by getting the old Xbox adapter, I'd do it.