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So an actual hood vent will always perform better than an OTR microwave. They have more power and better capture space. I would do a hood first, and then if counter space is an issue, consider a micro. The best micro for this is the LG with the extend-a-vent as it is more powerful than traditional micro and works better. The model # is LMH2235ST.
For a hood that is low profile, powerful, and reasonably priced. Look at the XO model XOA30S. It is quiet as well.