Another advantage of the MX-850 over the MX-350 is that the MX-850 has a larger (and update-able) installed IR code base. There are some devices which the MX-850 can control which the MX-350 is unable to control and not even capable of learning.

If you now use the MX-800 you should like the MX-850 much better than the MX-350, the MX-850 improves upon the MX-800 while the MX-350 is a noticeably lesser remote. I own both the MX-800 and MX-850, I owned an MX-350 in the past.