I've done both - I prefer Rowhouse. They teach you proper rowing form, and their rowers are far better than the rowers at OTF. Most people are rowing wrong, they think you just pull, or that the higher setting means you're getting a harder workout - which it totally false.
I also like that the instructors tend to do it with you, so you can watch their form and rhythm.
I found OTF kind of pointless. The treadmill stuff you can do on your own, outside. The strength training portion isn't very motivating. The rowing is where you can benefit most, but again, they don't teach you how to actually row at OTF.
Rowhouse became out of my budget, and I also moved away from a studio. But honestly, I enjoyed it a lot. If you haven't tried it, I'd give it a shot.