What's the difference between the galaxys2 and i9100 CyanogenMod builds?
There's no difference. The device was simply renamed within the build system between the release of CM9 and CM10, as the CyanogenMod Wiki page for the i9100 notes:


  This device used to be called galaxys2 on get.cm, so older builds of CM can still be found under that title.


So builds of CM9 (or earlier) will use the galaxys2 moniker, whereas builds from CM10 onward will use i9100.