The advantage of MS is that you can make a number of requests that you will not find when doing online check-in. Also, the online request sections were created mainly for cash paying guests and some do not even apply to DVC. For example at AKV Jambo, one of online requests you can make is high floor, but it is useless to waste a request on that because all DVC rooms there are on the top floor.
As far as whether one method provides more success than the other, that is an unknown. Success likely depends more simply on the status of things when you are being assigned a room and the breadth or narrowness of the request you made, e.g., if you ask for high floor, that usually encompasses a lot of rooms and thus a higher chance of success, while at the other end of the spectrum if you ask for a specifc room number your chances are much less.