I have written reviews on Amanjena (stayed 2017) and Mandarin Oriental Marrakech (stayed 2016) on my blog. I'm actually half-way through writing one on Royal Mansour (stayed 2017, directly after Amanjena) and a comparison between Royal Mansour and Amanjena....it's just God of War is far, far more interesting.
I said it at the time, and as we're heading back to Marrakech in September I can confirm it: I wouldn't return to Royal Mansour. It is one of the most spectacular buildings I've ever seen, but I really hated the room/riad. It may have a large square meterage, but it does not feel spacious due to the endless number of floors, most of which you will never use. As Groombridge said, the plunge pools are absolutely freezing - so perhaps perfect for you, but completely unusable for us.
I feel Royal Mansour is a place you go to have been, kinda like Burj al Arab - just this one has class. I can tell within the first hour if I won't like somewhere, but it takes a few days for me to realise if I'll love it. 1 night at Royal Mansour would have been enough for me to form my opinion of it, as nothing much changed between feeling just a bit underwhelmed by it all, once I got over how much the designers loved this project...and money.
If you want to be central than RM is better placed, although having been on two different tours, I feel Marrakech only has carpet sellers to offer in the centre anyway. Food at both MO and Amanjena was disappointing, whereas at RM it was a lot better - although I still didn't love any of it and thought it was overpriced. MO had the better suite, although I prefer the classic Aman ambience of Amanjena.
I would rank my favourite Marrakech properties as:
- Amanjena
- Mandarin Oriental
- Royal Mansour