A KEY-Series interface will work for the B5 and the Polo. But if you're thinking of upgrading to a B7, then you'd be better off getting a HEX+CAN now 'cause it will work with all of them. The Micro-CAN interface is not suitable for any of the cars you're contemplating.
USB vs. COM: COM is slightly more better / more elegant for diagnostics. The only reason we make USB interfaces is to accommodate the plethora of new laptops which do not have COM ports. We used to say "If you have a COM port, get a Serial interface" but the question you should ask yourself is: Will your next laptop have a COM port too? Hence if your PC is up to snuff (1 GHz or faster, running Windows XP or newer) USB may be more future-proof.
See also:
http://www.ross-tech.com/vag-com/interfaces.html#choose
-Uwe-