If you like the looks of the 66, buy it. 66's were a mid range machine when they came out and they were made for at least 40 years if not 50, Singer made millions of them. It isn't the best for FMQ, but for sewing clothes and piecing quilt tops it is very serviceable. Attachments are plentiful since it is probably a low shank machine - unless it uses back clamping attachments (not such a good thing since the back clamping attachments are pretty rare and expensive.)
Featherweights are cute. I have one and I occasionally use it for piecing. Would I quilt a quilt on it - no. It has a small harp and a small bobbin. I also don't believe it is suited for the task. My opinion only.
My go to machine for FMQ is a Singer 15-91. Durable, quiet running, and uses big class 15 bobbins. My second choice for FMQ would be, as others have said, a 301. The downside to the 301 is it uses the same small bobbin as the featherweight. It wouldn't be a deal breaker - who can't wind extra bobbins?!
I can't comment on which machine to use for sewing clothes because I don't sew garments - I know all of the ones you have mentioned were made with a seamstress in mind - so they would serve your purpose, unless you need zig zag or fancy stitches.
If your primary goal is FMQ, stick with a vertical bobbin machine. Learning FMQ is frustrating as it is - don't add to the mix by using a machine set up that isn't optimal for the task.