Main differences are speed and price. Wiggler is slow cos the protocol is done in software on the PC, but cheap cos the schematics are here on Elektroda, and you can build one for, like, $15 at the most...
Wiggler tools are also free.
Commercial JTAG-ICE units have a hardware protocol controller usually done in an FPGA, at 40MHz JTAG clock speed or something. (wiggler is i think about 200kHz). Also commercial tools normally need the associated software also pricey.