I have a Para and a Doug Ritter in Large. There are good points about both, but I would lean towards the Ritter, mainly because of the blade shape. If it had G-10 handles, it would be hard to beat! The Para is a nice knife too, but something about the compression lock I can't get used to. You really can't go wrong with either one, I guess thats how I ended up with them both! Good Luck!
I don't have a Ritter, but I do have a 551 Griptilian and a Paramilitary. I prefer the Paramilitary's ergos and find the longer handle to be an asset as it allows for more grip positions. I also prefer the G-10 to the plastic.
For me the compression lock on the Para actually works very much like the Axis lock on the 551. You hold the lock open and can flick the knife open and closed with either design. I find both very easy to use.