The biggest advantage to Dell's setup here is that the Venue Pro will be much more "exclusive" to those who buy it - for those that care about that sort of thing, that is.
Equally amazing is that Dell seems to offer their phones cheaper than T-Mobile's subsidized price and contract-free as well! Hopefully, that pricing strategy does not change with WP7 - but being that they haven't established themselves well in the phone market yet, I think things may stay relatively the same all around
It may also be free of T-Mobile branding which is always nice - once people figure out how to do a sim unlock for WP7 then we're good all around (I'd imagine that, being a Tmo exclusive launch phone, it would be carrier locked, otherwise it wouldn't be exclusive
Disadvantage is not being able to play around with the phone in the store, more of a hassle for return & replacement, calling India for support* (unless Tmo offers some sort of basic support for Venue Pro, I'm not sure how it works with Nexus One), and explaining to friends how to buy one if they want it and then the obligatory "I'm not sure why T-Mobile doesn't sell it" statement and then questions about how to buy stuff online and if it's safe to use a credit card *sigh* and listening to them complain if they have a problem and don't know how to send it back in even though return information is posted all over the documentation they didn't read when it arrived in their mailbox. (Can you tell I've worked customer service before?)