... unfortunately I have no experience in driving them but I did read up them. V8 is the sensible choice since it is lighter (not by much, 25kg), more fuel efficient, has a better engine sound (according to different car magazines), more agile to drive and it is cheaper to buy and to run. It shares the twin turbo V8 engine in various guise with the Audi S6, RS6, S7, RS7, A8 (V8) and the S8. The older W12 is definitely a more special and powerful engine only sharing it with the A8 (W12) and in the old days, the VW's flagship Pheaton.
Speaking of VW Pheaton, the 2nd generation Bentley Continental that is currently available still uses VW's D1 platform. First used in 2002 in the VW Pheaton. In short, it is rather ancient. However, in 2018 Bentley will launch it's next (3rd) generation Bentley Continental which will share 2016 Porache Panamera's MSB platform. The W12 engine will carry over to the new car and some rumours suggest the new continental will use Porsche's 4.0 V8 as well or they can continue to use the Audi's 4.0 V8, I don't know. If you are not in a rush, may be the next generation Bentley Continental GT will be worth the wait.
note- I wrote everything on top of my head. If I got any facts wrong, feel free to correct me=)