I picked Starwood over Marriott too because of the specific properties I wanted to go to (mainly WKORV/WKORV-N over Marriott's Maui Ocean Club).
This year I traded with a Marriott owner at the Marriott Waiohai. After comparing notes, I am happy to be at the Starwood property. I feel like Starwood has a bit more flexibility and since they have properties where it interests me most, I don't mind that they have fewer properties than Marriott. This can make a huge difference in your preference so I can understand why the availability would sway others in the opposite direction that I went.
Two things struck me at the Waiohai:
1.) Even though I had a room next to the "Elevator Machinery" I was told in no uncertain terms that it was not possible to switch to a different room category even if there were rooms available in your own category. The manager didn't just say sorry, we're full and there are no rooms available (I would have believed her since it was July), she said he wouldn't even look at what was available since my room category was full and it was Marriott policy to provide only the category purchased. I was also told that Marriott Timeshares are not hotels so they don't try to do what hotels do when there is a room problem. At Starwood I would expect the front desk to do whatever it can to fix an issue with a room, including upgrading to something better, if that is all that they have. I know sometimes this isn't possible, but at least they will try and not categorically go by the rule book. My exchanger even got an OF unit at WKORV-N (I own IV there).
2.) After our respective stays, the person that I exchanged with tried to reserve her week at exactly 12 months out, 6 in the morning (or whatever time she had to call Marriott from California) and was unable to get the week she wanted. She only owns one week and apparently those that own multiple weeks get to reserve earlier than she can. Also, the owner preference period seems to be only a few days. I called about a month later and was able to reserve my weeks starting in July with absolutely no problem. Hopefully Marriott owners will chime in here. It just seemed to be much more difficult to get something at your own home resort than it is with Starwood, especially if you don't own multiple weeks.
3.) The Waiohai was strictly a timeshare unit. I don't know how other Marriott resorts are. They did not have anyone to help with the luggage and only if you got lucky could you find the carts to take your suitcases from the parking lot to the room. There were a lot fewer staff going around the property than at Starwood. tarwood also rents their rooms like a hotel so they seem to have more services avaiable to guests. There are many more people available to povide concierge services, the markets seems better stocked and less expensive. The front desk staff was friendlier at WKORV.
4.) The Waiohai doesn't have lock0offs. Thus, you have to use your week all at once and you couldn't stay two weeks in your unit. The availability of the lock-off makes a huge difference to me in the flexibility to plan my vacations. This might just be something about the particular resort so double check.
But we will like sell those points as the kids get older (they are 8 and 3 now).