Todd:
I think you will be happy with the 430/420 combination.
Other than the resale and the uniform interface considerations, it is good to be able to fly a GPS course on one Garmin and look at the localizer on another. If you switch the Avidyne to look at the #2 Garmin (the 420) you will have the GPS course line on the Avidyne assuming that you have set automatic crossfill between the Garmins, a good idea (from #1 to #2 but not from #2 to #1). I am not sure you can make the autopilot fly the GPS course off of the Garmin 420 without an extra switch being installed, but that might be a good idea.
It is nice to be able to see that the localizer and glideslope are in the right places even when flying a GPS course to the IAF.
If you can arrange that, you have the same utility as a 430/430 combination.
As to GPSS, get it. Whoever said “it can fly a course as good as you can” is not my experience. In my experience, it is much better than I can fly. It anticipates the turns and takes into consideration, I think, groundspeed and always hits the new course dead on.