We went to Amansara when it first opened and enjoyed it as a base to see all of the amazing sites. It offers a wonderful cultural experience, but is not really what I would call a resort as it is in a busy area. It has the lovely Aman rooms, nice pool, etc. we went in the November timeframe and had good weather. March is on the border weather wise and could be iffy. We loved Amansara but really went to see the sites which are amazing and well worth seeing!
We went to Amantaka in January and loved it. Although it is also in the city, it is large enough and contained enough that it does have more of a resort feel to me. We did the Mekong river cruise on their private boat and it was absolutely the highlight of our stay. We went in the afternoon and had a wonderful light lunch and saw a glorious sunset on he way back. The temple we visited On the boat trip was touristy and IMO not great, but the boat itself was wonderful. Other than that we did the city tour - honestly could have been done on our own, and joined the Aman in watching the monks pass through the town for "alms" (offering) early in the morning which we really enjoyed. The aman does this in front of the hotel where it is not filled with tourists like the town center which is really nice.
We were only there for 3 days and could have stayed longer as I really enjoyed the property and would have liked to lounge around a bit more. We had a gorgeous private pool suite - one category up from the regular pool suite and I would highly recommend. I had a great massage as well.
If we had stayed longer there were trips to a waterfall and into the countryside that looked very nice as well.
In terms of which order to visit, that is a hard one. Cambodia has much more interesting cultural sites, but I think Amantaka is a nicer place to relax. Although I don't think of either one as a resort like Amanpuri or pulo or the resorts in Bali. We went to Burma after Amantaka which was fascinating ( although not nearly the aman like accomodations) and then ended our trip at Trisara to relax which was perfect! If I were you I might look at ending with a few days at a resort like this to really relax after the cultural things. If you have 12 days you could do 4 each! I think you might even be able to fly directly from Siem Riep to Phuket, but not 100% sure. Love Trisara and it is a great place to chill after a lot of sightseeing! Of course Amanpuri is great too!
Have not been to soneva kiri but that is another alternative and they have a private plane that picks you up at BKK airport. We are thinking of trying next year.