I know I'm going to sound repetitive here but.....
be sure to pack anything you need for the first 6 hrs or so, in your carryon bag. Even if you get your own luggage. People need to understand that in most cases, it is the airlines that lose the luggage, not DME! Once in DME's hands, the worst that may happen is that your luggage gets sent to a different resort...easily tracked. But, when luggage goes missing, or is delayed, it is generally the airlines at fault. But, it doesn't happen often.
So, yes, you can get your own luggage and have it put underneath the bus. But remember, although it says in all DME that gratuities are included, that is based on DME taking your luggage directly from the plane and taking it to your room. If you choose to get your own luggage, you should be prepared to tip the bus driver who stows, and then removes, your luggagem, and then the person from Bell Services who delivers the bags to the room.
The only time I recommend getting your own luggage is if there is a chance the bags won't get to the room until very late in the evening. Other than that, you are pretty safe in having DME transport the bags.