Both the Garmin 650/750 or the Avidyne 440/540 (or 550 which adds an AHRS board if you want an SAI built in) will do the job nicely. As stated above, preference very well may be a function of your previous experience. The Garmin interface is largely touch screen only based and more often than not needs to be backed out of one menu to enter another. They are also prettier.
The Avidynes differ in that they are both button and touch capable such that there are multiple ways to achieve input for perhaps 95% of the functions. They used to advertise that usually 2 button pushes, and almost never more than 3, will get you to the page you want. That variability can make your previous experience more applicable. If you have ever used a high level FMC professionally, their input flow simply feels more pilot friendly. If you have ever had to enter a long full reroute clearance in turbulence, you will appreciate having buttons.
One looking forward aspect you might have not considered: Avidyne goes to great lengths to make their equipment fully interoperable with a wide range of other manufacturers' avionics. Garmin equipment, while somewhat adaptable for use with other mfg. avionics, seems to often reserve complete functionality for only other Garmin products. Translation: Which will be easier to sell years from now should something new and shiny catch your eye?
Last, a BIG consideration: Check out Avidyne's FREE IFD 100 app for your iPad. Garmin has nothing like it.
Ken