From memory:
K03-005 - the most common 150bhp 1.8l 20v turbo from a Passat or A4.
K03-029 - turbine side is larger than above, so boost threshold is higher but flows better at higher rpm than a -005.
K03-073 - from a 190bhp A4, is a -029 turbo with a larger compressor. Kind of a 'K03s' for the longitudinal engines.
With the 268° turbo cam I think the common K03-005 is a great all rounder. The -029 is handy if you're happy to sacrifice a touch of responsiveness for a few more bhp and I quite liked it. The -073 is what I run now, biggest issue I have is getting the Forge actuator to control boost properly at 21psi without creep.