I own both , along with a Berry Morningstar.
The MD and the Vixen both shoot fantastic. I cannot for the life of me really discern a difference between them when shooting them. They are both smooth, quiet, neither have any shock. As far as length goes, 66" would work if you just wanted a little shorter bow, both of mine are 68".
If you are wanting to try a string follow bow , or you have in your head that they are more forgiving , then go for the Misty Dawn. BUT if you are even the slightest bit interested in maximizing performance why staying with a straightlimbed bow AND you want amazing forgiveness I HIGHLY recommend you read up on the Morningstar, talk to James and Ask a few of us who already own them.
I can promise you this much, whichever you chose, it will be a great choice because James does not put out a bad product. Fit, finish, ease of shooting and performance are just impressive.