FLM, exactly how I felt re the a900, it is too good to rubbish it much at all and served me very well for years. The files of the new one are very nice and the camera is liberating in other ways too - it feels good in the hand, the small weight loss helps, focus aids, dual slots etc.
While I want to get the a900 'semi-dissing' out of the way, the real strength of the a900 was as a kind of 'medium format lite' camera for its time, long before 24Mp NEX, the rise of Fuji/Olympus in smaller formats, and so on. It was clearly intended for great low ISO work with fine colour and tonality.
DxO sensor metrics make it clear that Sony have tried to counter SLT-centred losses by giving the a99 a largely undisclosed great low ISO performance - exactly the strength of the a900. But you also get good mid ISO, with much more appealing noise integration, if there can be such a thing, opinions will no doubt vary.
There were interminable discussions about what ISO was best on the a900, because users disliked the noise characteristics, and the main debate was that either ISO 200 or 320 was best. I found 200 and especially 160 much cleaner but will use 50 and 100 whenever doing that kind of work (tripod) on the a99, conditions permitting.
I (and others no doubt) would be interested in anything you can offer on the 'mirrorless a99' experience - I rate personal opinions higher than many here, as there is nothing like looking at good RAWs. thanks very much!