You don't want a FID bench, it's extremely uncomfortable. I was just using one the other day to warm up and kept getting cramps in my hips trying to put my legs around that seat. The height is also a valid concern, although plenty of people use the Ironmaster Super Bench (that's what I own for F/D
) @ 20" high and make do.
If you're going to have just one bench, then get the 3100 (or better yet the Elite FTS or Rogue utility benches). If you're going to have two, then buy a utility bench and then do whatever you want between the two options above.