I've only biked off-road once, so this is a first impression.

Road - I can look around a lot. I get a good workout, but I can also relax. (I work on a lot of paperwork, so relaxing my eyes feels really good to me.) I look at the road down a ways, then close, then look at birds, or chat, or whatever.

Off-Road - I had to look down at where I was going a lot more. It was challenging in a really different way, but not as relaxing for my eyes. I could see getting a really good workout faster.