Definitely get the V8S. I have extensive experience with both vehicles, the W12 engined models have a much more traditional suspension whereas the V8S has a supple and yet sporty suspension. Power difference is negligible as far as my application in congested California goes. The W12 engine is smoother than the V8 though, noticeable, but not that the V8 can be faulted.
V8S is definitely the way to go. I'd take the V8S over a W12 even if there was no extra expense for the W12. You definitely want the sportier and more fun to drive V8S.
A lot of salespeople state the W12 drives significantly heavier. While I understand that feeling, the actual weight difference between the W12 and the V8 is only about 100 kg. The differences in handling dynamics is not actually due to the weight of the car but due to the way the suspension on the V8S is tuned.
Hope this helps.