Combo non venting units are terrible from my experience. You're forced to do only small loads, those small loads will take longer on average than separate stackables, and the worst part, when you get your clothes out after a couple hours they'll still be wet so you'll be forced to hang them to dry out the rest of the way. Because of the small loads you'll be doing this a couple times a week. And I never got a sense that my clothes were as clean as they should be.
Maybe there's some good ones out there and the complex I was in just bought a dud brand, (they seemed to break down and leak a lot) but if at all possible I would recommend stackables if there's an outlet for it. You can vent into a little detachable bucket if need be.