Is the new Di2 really that much better than the current version? Do you need that extra cog? Do you feel it's worth it having to buy new wheelsets or modify existing ones so you can have that extra cog? Are you planning on internalising the battery?
I'm building a new bike up as well and I'm just going to go with the existing Di2. Sure, the new Di2 will be released in the next 6 months, but I won't be able to afford it until probably another 12 and by that time I'll most likely be building a new bike anyway! Plus, think of the bargains when people move to 11 speed!
im with u... im more than happy with the exiting DA Di2... has many advantages... one being is hackability, which will be largely eliminated with the new DA Di2 (as it has been with Ultegr Di2)
I plan to buy up on the existing DA Di2 now that prices will come down... having an extra gear, and the ability to program shifts doesnt rally bother me much to encourage the very large, extra $$$$ outlay, not to mention the wait....