I'd go for the 15 any day of the week. I love the vertical bobbin. The horizontal drop-in bobbin may be easier to load, but I don't think it sews as well. I NEVER have any problems with thread or tension on my Singer 15 machines (I have several sub version: 15-88, 15K90, 15-91). 66's are fine machines, I just much prefer a 15. One potential downside--the harp area on a 15 is slightly smaller than a 66, but it's never been enough of a difference to sway my opinion of which one is the better machine.