I can't speak to the A99, having never shot it, however I agree with Barry's comments on the 24 APS-C Sony. Past iso 400 it's just too noisy in most situations. The noise is more like film grain and doesn't have the banding, color smearing I saw with Canon until the 6D. However it's still pretty harsh.
I switched to the Fuji X-E1 for mirrorless and have found it's image much more rewarding than the Sony's in most situations. The Sony at 100 is excellent but with wide angle glass, (I tried all but the Zeiss 12mm) you end up cropping into the shot to around a 20mp image due to corner softness and detail smearing many times. The Fuji has less MP, but I feel you get more from them.
The upcoming Nex-9 or whatever it will be called might be a deal breaker for many as it's already been shown that Sony can produce a very clean full frame 24mp chip, Nikon D600, Sony A99, Sony RX1. Placing that chip or an upgrade to that chip should make for a very competitive camera. I am wondering where Sony will price the Nex-9 24mm FF camera. Hopefully not at the same price point as the RX1.