i am getting ready to install pex in my 2 bath 3 bedroom house using the manabloc manifold and am thinking i'd use 1/2" out -- is there any reason i should consider 3/8" out instead. thanks for any advice.

The larger the pipe the more water is delivered to the fixture. 3/8 is fine for water supplies to fridge and undersink drinking faucets. But not for kitchen and bath fixtures. Ideally you should be using 3/4 inch where ever you can. But with manifold 1/2 should be fine to the fixtures.