SugarSync is just my preference. By no means do I suggest it for everyone. It's still much quicker than the iTunes method, because it requires no action from the user on the computer where Calibre runs. Any EPUB that shows up in Calibre, automatically also shows up as a download option for you on your iPad, in the SugarSync app.
Dropbox is more suitable than SugarSync for Marvin users, because Dropbox is integrated directly in Marvin. You have a dedicated Dropbox button inside Marvin, and can load dozens of books into Marvin that way, in a breeze. Again, much faster than iTunes. (I only use SugarSync for books instead of Dropbox due to space concerns.)
As to the direct option to access Calibre's OPDS catalogues from inside Marvin... you can't really compare the speed of that type of access to the iTunes method, right?