The 12.50 is of course much bigger, especially inside, and it will certainly be a bit more forgiving when loading 4 or more adults and all the gear for a longer cruise.
We did not sail the 12.50 since it was due for the boat show in La Rochelle. But everything we heard and read seems to confirm that she is just as easy to sail as the 10.50. Which means: very easy.
The directional stability when motoring is even better with the keel up than down, also at very low speed. Turning is the problem and the foil in horizontal position is also much less efficient to whitstand windage on this very lightweight hull. ...
BB74, I think Eric is right. I would expect that the major difference would be on the load that the boat can carry without affecting too much the performance. The 12.50 can probably carry almost the double of the load the 10.50 can carry. The extra boat mass will make it probably a little less nervous than the 10.50.
Regarding directional stability, when I have tested the Opium 39, the first thing the guy that went with me has done, still in the marina, was to lower the keel. It seemed odd to me and I asked why? He told me that the boat was very hard to steer with the keel up. It seems that the problem came with the territory (two rudders, swinging keel, large transom boats). I would not have many hopes that with the 12.50 it would be any better.