Replace Over the range microwave with hood
Manufacturers will generally direct the installer to minimize the number of elbows and transitions, example:



There should be no problem installing it whichever way is easier for you as long as you follow recommendations and restrictions in the manufacturer's installation instructions and any other direction from your permitting authorities (if you are pulling a permit...)

I would recommend that you retrieve the installation details from your chosen appliance supplier or manufacturer, before you actually purchase the unit, so you can ensure that you are prepared to install it properly.