Nick,
I just bought 650 this winter after loading on a 550 for years. I would get the strong mount. I'm not sure if you are aware of this or not, but Dillon makes two strong mounts for the 650. The 550/650 strong mount and a strong mount for the 650 only. The strong mount for the 650 is a couple of inches shorter. Basically, I think it is personal preference, but the strong mount, in my opinion, will make the press sturdier.
Another opinion of mine- I wouldn't get the 650 without the casefeeder. It is wonderful to have the machine feed the brass right into the shell plate. As far as how many cases it will hold- I think that is a function of the clutch mechanism. If there is too much weight, it will slip so the motor doesn't burn out.
Even with height issues, I still think it is worth the trouble to get the casefeeder.