the mila material is better, faster to render, better SSS. Not only that it's the future of mental ray, the new render pass system only works with the mila material. Layering materials as opposed to just having one giant one makes a lot of sense try it

As for nuke vs photoshop, well PS has no true plus operation the closest it comes is screen, so it's actually not possible to put something back together in PS (I forgot linear dodge, just a reminder to not post anything before coffee), it will do for a quick job but it's not a true compositing application. There is as far as i know no premult and unpremult in PS so handling of alpha channels for render passes is horrible.

Don't get me wrong you can use PS for a quick job, but if you want to do it right move over to nuke or another compositing app.

So maya passes -> nuke composite and colour management -> photoshop if say you wanted to do a paint over (can also be done in nuke but i prefer PS to paint in)

Hope that helps
Dave