I would not rely on either of those for requests that your really want. If you have definite room requests, make sure they are listed on your reservation - either by calling in to add it or having your
Travel Agent add them for you. As many others have stated here before, all room requests are just "requests" and not a guarantee that you will get what you ask for.
If, when you get there and the room doesn't meet your needs, you can ask for a different room at the front desk, but again, there needs to be availability for them to be able to do it for you.
Personally, I always use the TP request option, however I understand that those all go to a central room assigning group now, and sometimes it works out and sometimes it doesn't. I don't bother with the On-Line check in requests because they are so limited they don't really make any difference. But I always have my TA add requests to my room reservation.
Again, not to sound like a broken record, the requests are only requests, and I more often than not do not get the "perfect room" like I dream about for months ahead of time....