I play both and switch back and forth several times during a set and sometimes mid-song. It's not that difficult to learn at all. I was early mid-30s twhen I picked up the mandolin.

And I just use the regular mandolin tuning GDAE. It's easier to find the scales since the strings are tuned in 5ths like the fiddle. Now if I'm improvising a solo I actually prefer the mandolin, both for the ease of getting around in a particular scale and the higher notes cutting through better. Of course the higher tension double strings pretty much preclude any string bending techniques.

The toughest thing about it is the smaller scale like Ballyholme says. Sometimes mt long fingers get pretty cramped up on top of each other, but it just takes a bit of practice and thinking about your technique to avoid problems.

Check out the Mandolin Cafe for some excellent tips and lessons.