Have both Ultegra Di2 and Red eTap 11 speed. I know SRAM have a bad reputation for tolerances (I believe Hambini uses the S-Bomb a lot to describe it
) but knock on wood so far mine seems to be going on quite well. Both shift very well, with maybe an incremental advantage to the Di2 in terms of smoothness and snappiness. Riding one after the other I can sense a smidge of difference (or maybe it's in my head) but I can honestly say if I only had one I wouldn't miss the other.
The major differences are well known: Di2 has better shift programmability (although this may have changed with the Red eTap 12 speed, I don't own that version). Di2 has the one battery in the seat tube which is charged through the junction box, whether that's in the bar-end or under the stem or integrated in the frame somewhere vs. the four batteries that you have to charge on the SRAM (two in the derailleurs, which charge via the supplied charger, and 2 in the shifters which are the coin batteries (2032s?). I've not had to swap out the 2032 but I have had to charge the derailleur batteries once, and it was fine. I don't consider it much more onerous than the Di2 but maybe when I have to change the coin batteries I'll sing a different tune. One cool thing about the SRAM derailleur batteries. I got a warning the rear one was low but not the front one. I swapped and both registered as normal (I switched back and got the same warning, so this wasn't an error). I assume that's because the AI assumes you're shifting the rear way more often than the front, so what is "low" for the rear D is different than the front D. So in a pinch, if you get a low-battery warning and you're far from home, you can always swap the front and back batteries and pretty much get home no problem. Also the wireless is very cool and reduces the amount of visible wiring too--so more aesthetically cleaner look and possibly a minuscule wattage savings. Of course if you having fully internal/hidden cabling this isn't an issue.
I saw the new Force seems to be close to the Red (incurring a weight penalty) and Ultegra is pretty much the same deal with Dura-Ace (95% of the performance plus few hundred grams).
Bottom line I'd ride the bikes you're interested in and get the one which feels best to you. I don't think you can go wrong with either the Force eTap or the Ultegra Di2. I'd let the rest of the bike be the deciding factor (geometry, comfort, stiffness, compliance, wheel quality, fit, ride position, etc.).