I personally own the 10 inch and am happy with the choice. I'm a rather small (5'6") man with a history of a broken hip. After a few nervous moments when I first got the scope, I'm now able to move it easily. I might also have been able to lift and mount the 12" now, but I doubt I will have the strength in 10 years.
The 12 inch has 44% more light grasp and a slight edge in resolution, so those are major considerations. However, it also has considerably less "swing-through" room for accessories both on the eyepiece end and around the tube. The 12" will allow the standard diagonal/focuser/eyepiece to swing through, but has less room if you're stacking other things. This only matters if you're looking straight up in altaz mode, or at the pole in polar mode. The 10" and 12" models use essentially the same fork, except for the extra handles on the 12", and this means that there's just a bit more room for accessories on the 10" model.
One more consideration is the vehicle you have for moving the scope. The tripod for the 12-inch is considerably bulkier. Don't expect to fit it into a tiny car.
If I had a permanent observing site, I would have gone for the 12-inch, but as it is the 10-inch is at my personal weight limit, and the small difference in weight is critical for me personally.