I ended up buying both. While I am sure the seneca adapter works great, I felt that festool made 2 different units for a reason. Yeah, it cost me significantly more. However, I hate screwing with things and I felt that I had to go through to many hoops (adapters, shims etc) to make the 700 do something it wasn't designed to do for 90% of my work. I just felt that the money spent on the seneca adapters would be better spent on the 500. I don't regret the decision.
FYI, I'm a hobbyist, with a lot of other hobby's.