Haven't stayed at either one but from what I've read...
Horizons at Orlando is a brand new resort in active sales. It's geared toward families with kids and has a very popular main pool area w/ fun pirate ship, water play area & kiddie slides. Organized children's activities, playroom, high level activity, etc. 2BR villas sleep up to 8 people.
Royal Palms is much more established, long since sold out to owners, w/mature landscaping. It's on shared property with MOWC, which has a fabulous waterfall pool (several pools, in fact) w/longer slide and a much more adult atmosphere... including several restaurants, shops, indoor pool, meeting facilities & onsite golf. The RP resort itself has a quiet pool area, shuffleboard, tennis and a more laidback environment. 2BR villas are larger but sleep up to 7 people. They include a double jacuzzi tub in the MBR.
Reviews of both are generally very positive. Both are located pretty convenient to the theme parks, just a 10 minute drive. HTH.