We are regulars there - we have stayed at both HRH and RPR - they both have plus and minus points.
HRH is livelier - more families with kids - great theming, lots of memorabilia. But - the car park is tiny, you need to valet park if you get there early in busy season. The pool is a playing pool rather than a swimming pool - it is too shallow for a proper swim, it has sand in the bottom and is busy. You can walk to the park in 10 min or get the free water taxi.
RPR is quieter, more of a peaceful tropical theme going on, families with young kids, and the pool is bigger and deeper, the car park has plenty of space - but it is also used out of season as a conference centre. It takes longer to walk to the parks - 20min+ but there is still the free water taxi.
You can check in any time from 6.30am - and can use the express queue access on both check in and check out days if staying one night. Parking costs about $25 a night including tax - one night covering both days. You can leave luggage with bell services on both days too. When you check in - if early the room may not be available - they will text or email you when the room is ready.
Both hotels are fab, really comfy beds, nice power showers. You can find either of them is cheaper depending where and when you book. When you do book, if you book direct then tax is not included in the headline price, some websites (BA.com etc) DO include tax, some do not. Some sites also allow free cancellation or changes, others do not. So make sure you compare like with like.