Does your 66 treadle use back clamp or side clamp feet? I have a 66-1 red eye and my MIL's 66-1 lotus and both seem to be strong sewers with excellent stitches. I have never sewn on a 27 so I can't compare. My only thought is, if your 66 takes a back clamp foot, be sure you can find the accessories that fit it. There are back clamping feet out there, but in my area they are hard to find.
Good luck with your decision.