Jan,
I've been to Ambroseli while we were on the WC and the animal viewing isn't nearly as good as other areas. We went to Mateya (not Regent sponsored) for 2 nights and it was fantastic! You might consider doing something like that for a fraction of what RSSC charge$. If you feel more comfortable going on a RSSC safari than I'd highly recommend going for at least 2 nights. We have friends who went to Elephant Safari - Camp Jabulani and raved about it. For sure, you can't go wrong with Singita but that is probably the most $$.

Have you talked to Ngaire about these options? ...she's the African Queen and has probably visited all of the camps you're considering. She's the one who recommended Mateya for us and she was right on!!!