Since you have the 128, I say use that. You will only be able to use it in c64 mode with the SuperCPU. IIRC, GEOS, Contiki and Wings all work in the c64 mode of the 128 so you won't be able to take advantage of 80 column mode.
If you are in the mood for these 8-bit hacks then you should check out
this link. A hard drive on your 64/128.
Hope this helps. ;-)
