I'm no expert on this but the important thing is the vent outside itself. I mean, having the right size ductwork, venting with the shortest ductwork as possible, with no turns or as few turns as possible, is what matters. It's the ductwork venting system that matters, not whether or not the OTR microwave is convertible or just simply outside exhaust.

Thanks, funkycamper.
I think it will be vented directly out the back of the MR - it's an exterior wall. I just didn't know if there was a difference in the MW's internal venting system between the two types - ie: does the convertible turn into the same thing as the outside only (are they the same systems.)

Edited to add: Yes, in your floorplan it's a very short duct that vents outside with no turns so the main issues you need to concern yourself with are CFMs and sones.

CFM rating (cubic feet per minute) is what the fan is capable of drawing out of the room and venting out. That is going to vary on model. The higher the better. I believe the highest rating you're going to find on a micro-hood is around 350-400. Also, you want the fan to run as quietly as possible so you're more likely to use it. You need to find out what the sone rating is. Sone rating is a sound measurement; different than the decibel rating. I don't claim to understand it. But the lower the sones the better. Of course, if you're using a stronger fan setting, the sones will always be higher than the lower fan settings.

If someone has found a way to explain what the sone ratings mean, I'd love to hear it. I researched all over and found minimal explanations. I was hoping to find a chart that told me that 30-35 sones is similar to the sound level of blah-blah, 35-40 is like blah blah, etc. Couldn't find anything to give me any guidelines. Frustrating.

I was glad to find an appliance dealer who had exhaust hoods hooked up in their showroom so I could turn them on and hear them in action. This allowed me to choose one with the appearance and features I wanted, the CFMs I wanted, at a sone level that seemed quiet enough for us to use regularly because it's not too loud and obtrusive. Without being able to listen, i would have felt lost and had to make a by-guess-and-by-golly purchase. I suggest you try to find a place that also allows you to listen to how loud the fans are.

Anyway, that is the basics and the best tips I can give you. Hoping someone with more expertise jumps in and expands on my statements or corrects them if I'm mistaken about something.

That's very helpful. Thank you! I did a Google search & a search of "Advice" forums & your explanation gave me more of what I needed in an evening of searching.

LOL....well, probably because I did my own evenings of searching. ;)