Short & sweet reply from a long time RI La Plage owner.
1) I would stay in any La Plage unit over the La Terresse. The view and amenities as well as feeling apart of the resort would make a big difference to me. As another OP suggested, hit up Richard March at RI owners website for rental information. He is honest & trustworthy....and he is Canadian.
2) La Terresse can use the beach....if there is enough sand to make a usable beach area. The sand comes & goes which means that La Terresse owners may be limited to their pool area or traveling to Mullett. Or walking down the street to Maho beach at the end of the runway.