Well I used to think that Saito engines were ugly things -- that awful big cam box up front and those awkwardly angled pushrod tubes just looked wrong.
The rocker covers and head finning also looked so 1920s that I thought they'd be okay for an old WW1 scale model but that's about it.
Then I bought one.
Man, when you see them in the flesh (as opposed to all the pictures I'd seen) they are *beautiful* works of art.
But from a pure mechanics perspective...
OS 4-stroke engines are fine (although I've heard a few tales of woe about some of their more experimental versions (EFI anyone?) and the FL70 appears to have a slightly mixed reputation.
One of the guys at our club has an OS120 and it's a nice powerful, reliable workhorse -- nothing wrong with it at all.
I prefer Saitos -- but that's because I fly 3D and weight is *very* important to me.
One thing worth noting: unlike my TT91FS and the OS engines I've seen, my Saitos have *never* thrown a prop or backfired on me. They develop excellent power and have super-sweet handling characteristics on 10% nitro but liven right up if you want to hike the nitro to 30% or so.
Another big plus is that Saito parts seem pretty cheap. I replaced both pushrod tubes and pushrods on my SA100 after totally burying it in the dirt and it cost a *lot* less than I expected.
Your aircraft type, wallet, flying style and personal preference should be the deciding factor when chosing a brand. Either OS or Saito will make you very happy if you learn how to tune and care for them.