Having stayed at both, mount nelson (couple times) all the way. Get a garden villa - almost direct access to the adults only pool. O&O huge, impersonal and personally we always avoid the waterfront area. Touristy, dirty and not very nice. If you want generic, go there.
If you stay there, eat at Nelson’s eye, bit of a dive but very good local meats and charming service. Gets keys to access through the side gates.