Convenience: Sierra Tower is most convienent to DTD, DL, and DCA, but most (if 9if not all) of the rooms are view room. Marina Tower is second most convenient to DTD, DL, and DCA. It is also most convenient to the lobby and shops, and is convenient to the Neverland Pool. Bonita Tower is the least convenient.
Views: As noted by other posters, the Marina standard view rooms look out over the front entrance and parking lot. However, you are able to see DL off to the right, including the fireworks. Bonita standard view rooms look out over parking areas toward the PPH.
Summary: If you are booking a standard view room and want convenience, choose Marina; for views, choose Marina.
To increase the likelihood of having your request met, call ahead 2-3 days before arrival and make you request (even if you make the request at the time of booking).