It's kind of personal preference. Do you mind charging each derailleur and shifter individually? I think the wireless nature of etap outweighs the benefit of charging the battery from the junction box only. The benefit to di2, especially in a TT, is syncroshift. You only need to use one shift lever.

Personally if they're the same price I'd definitely choose etap.

I have both: Di2 on both TT and road bike. Then eTap on (2) road bikes (yikes, did I really say that?)

To be honest, I prefer the way that Di2 performs (shifts faster and cleaner). The advantage with eTap is no wires. Which is why I have it on my Ritchey Breakaway, and then on the Madone I just set up (and part of that was just to make it easier build.

I'm able now to adjust to to the different shifter pattern pretty seamlessly. Not having tried it on the TT bike, not sure which pattern I would prefer. My guess is that like with road, I'd adjust to either.

Is there a big weight difference between etap/di2 and the mech versions ?

honestly, I couldn't tell you (though I'm sure Google can). I'd imagine the battery is going to make it a little heavier, but I'd guess it's negligible.

Personally, I will never go back to mechanical shifting on any bike that really matters. I rode all winter on my CAAD10 with 10sp Red, and it was really nice to get back on the good bike!

I have Di2 on my TT bike and eTap on my road bike. Di2 on the TT every time. Synchro and the integrated shift/brake levers are ultimate. eTap for the road bike win, every time.