OK, the Contax 100mm on the a900 'looks' very nice, an elegant facsimile of reality, like a soft low key photo. Focusing in soft light (loungeroom) was enjoyable but 'inexact' and slow; actual target focus took a few back and forth turns and the one I settled on was maybe 40% a guesstimate, the focal plane might easily have been 4-5 cms in either direction.
The a99 EVF image was both less appealing marginally yet very 'work ready', with maybe a 2 stop brighter image (EVF is set to low brightness, I prefer a softer look) at correct exposure. It showed good peaking highlights (glasses, hair and eyes) - to give you some idea of the light levels, this composition gave me 1/30 sec and f3.5 at iso 8000, or 1/13 sec and f5.6 at the same iso - and focus peaking showed effectively at f5.6.
Focusing took perhaps 25% the time taken on the a900 and I knew I had it right first time. The histo and SS told me I had exposure right. This type of shot is a bit of a stretch for a 100mm lens but I shoot a lot of these exact light conditions with a Summicron 50/2 or 21/2.8, so you see why I say its a big improvement on the a900 - for this kind of situation.
So two big wins: (i) easier, faster, more accurate focusing; (ii) ability to focus at f5.6-f8 in poor light if ever needed, as that is simply beyond the a900. This also gives an estimate of DOF via the peaking and the image appearance, made more effective by the consistent EVF lighting.