I had really good experience with a little G310 on the back of my Giro 26. I didn't need to climb steep hills but did want more speed, so I actually used the version intended for 20 inch wheels. It worked well, as would the standard winding, I suspect.
I'm not using a motor now, but if I were I might consider the slightly larger offerings. I don't really see the need for the bigger GMAC or Grin motors for this application.
Most important on these bikes: mount the battery low and centered on the frame, preferably under the boom.
Enjoy!