For me a big difference is the resolution.
The Oregon is 240 x 400
The GPS Map 78SC is 160 x240
When I bought my Oregon 4 years ago the resolution was a big concern for me. The difference between Oregon and the 76/78 handhelds is huge, and to this day I don't know why so many people select 76/78 over it.
Whichever you buy, the most economical way to mount it (if you have a pedestal guard) is this:
In addition to the one I have on my pedestal guard, I have an extra one that I take on charters with me. At the end of the week, I just cut the straps and bring it home.
Since the Garmin handhelds do not float, it's important to mount them, especially if you have an open transom.
In dry weather, I use OpenCPN in the cockpit. I run two instances of the program on my Netbook. One window is zoomed in for detail, the other one is zoomed out for big picture and AIS targets.