the only diff apart from cost is that an architect , coming from an arty background might be able to design something a bit nicer. ive seen loads of plans designed by draughtsmen-engineers and most seem very boxy and functional , like something lifted straight out of a leaving cert tech drawing class.
on the other hand, from a business contact in the concrete industry ive heard that a lot of architects can be clueless about structural issues/ loads etc and design stuff that cannot/cannot easily be built.