I like the way OTR microwaves look, IMO it looks better than just a typical range hood. But, I don't think any of these microwaves vent very well. And most range hoods with the little five blade fan don't either. More noise, but little suction. In either case, I would want it vented to the outside. I think it should be made code for all kitchens to have the exhaust vented out. The recirculating filters don't remove odors very well and the one we had just blew the grease across the room. Not a big deal now as we don't fry very often.
The other thing I'm not sure about OTR microwaves is it would seem the grease would get in them and there's really no way to take apart and clean them like a regular range hood. I would think they would get nasty.