The next highest slots are on the safeguard seat. They measure about 19.5 or so. I think it has more room than the marathon. However, it retails for $400-450. Next in line is the radian ($200) with 18+ top slots. Should have a bit more shoulder room than the marathon. Both these seats have somewhat short depths in the seating area, though. For kids with long legs, this may be a problem. The regent has a nice deep seating area.
If you can wait a little while, the new recaro seat ($250 or so) is coming out and is rumored to have 19 in top slots. Not sure of the seat depth, though.
The regent really is a great seat, though. I have total faith in it's ability to restrain my kids and it is the only seat out there that will harness my kids the longest.
HTH and good luck with your decision!