if you can afford both, keep them. sounds like you (and your sis) can benefit by you leaving one machine there w/ her and keeping the other at your home. makes total sense to me! I don't see them as being duplicates of each other. The 6600 is a very nice machine but it does lack some of the refinements of TOL models...even TOL models with wide harps. The free-arm feature alone makes the Horizon desirable and i think it also has a larger harp area, correct? There are many, MANY more stitches. Its probably twice the machine of the 6600 so there will be plenty to love about it. Most of us have more than one machine. Truly it is more the norm than not! I understand buyer's remorse, tho. as i do also beat myself up after purchases, but you ain't married to none of em and can sell one later if you choose to do that. Give the 7700 a chance to prove to you that your instincts were spot-on when you bought it! We second-guessers don't usually part with our money easily so there's a reason you bought it!