I think the biggest difference is visually, the screen size and the Nav graphics. There are obviously a few extra features, but none seem to be killer (for me.)
Personally I didn't feel the need for the bigger screen, and the standard Nav already felt a bit on the steep side for me. I'm pretty into my tech but I've never been in my car and wished the screen was bigger, it does the job great.
If I sat in in a car with a Nav Pro I would probably want one, but not for the money!
I would recommend DCC for the money over the Pro, but it depends on what is important to you of course.
Word of warning... you may end up like me, I joined this forum around the time I put my order in, before I knew it I had added 4 or 5 options thanks to reviews and comments on here
I don't regret any of them though, whatever you go for - it's all put together brilliantly.