My Excel has separately ducted return air like the second picture. There are no exposed parts of the ac. My dealer suggested it as an option along with the padded ceiling to reduce noise, and it does. It is much quieter than the standard system we had on the motorhomes and other fivers we have owned. It is not something I would have even thought about had he not suggested it. As far as any down side, I can't see any. The system still works the same way.
Both our living and bedrooms a/c's are mounted this way. One thing we have noticed is the cooling and heating (we have 2 heat pumps) is more evenly distributed. It is nice for sleeping as the air is not being dumped directly on you.
