I know I'm going to be the odd duck out because everyone is attracted to the newest gee wiz **** as soon as it comes out, i'm not!
I would go with Ultegra mechanical, it's bulletproof, every once in awhile you do have to replace cables, but it shouldn't be once a year, I've had my cables on my bike since 2013 when I got the bike and their still fine! I do know how to keep in eye on that stuff so I'll be able to tell when the time is right to replace them; I know I'll hear the screams of those who replace theirs every season, and that's fine, but I've doing it the way I been doing it for 45 years and never had a cable break, just a bit of lube on the cable where it goes into the housing and they keep working great for a long time. So for those that will be screaming about Di2 advantage of not having to replace cables every year all I have to say is BS! And another BS call goes out to those that say mechanicals always have to be adjusted, again I got my last new bike in 2013 and so far I have yet to readjust the derailleurs, which is on par with my older SIS and Friction stuff, I never have have to readjust till I put new cables on and after about first 300 miles of putting on new cables it might need minor tweaking after that no more fiddling around with it.
Di2 systems are not without their issues, the battery only last about 2 to 3 years before needing replacing (which Shimano says is necessary that frequently, and the battery cost $120 or so, a set of Shimano Dura Ace cables around $60 and that's for the good stuff), which is a crappy battery since it may only recharge about 6 to 10 times during that period of time when rechargeable cell phone and tool batteries can go at least 500 times, but that's Shimano for you, and here's more that Shimano doesn't tell people:
https://road.cc/content/feature/253498-stuff-they-never-tell-you-about-electronic-shifting Rough roads can cause wires to come undone which is what happened to Thibaut Pinot and Peter Sagan during TDF races, while you don't have to worry about losing a race you do have to worry about how you're going to get home, but I think the new wireless systems will eliminate that problem but turn around and give you another battery to worry about. There has been a lot of issues with the system just quitting, here is what you need to do if it does:
https://www.reddit.com/r/bikewrench/comments/4pacpq/di2_stopped_working_completely_in_the_middle_of/ I'm sorry but mechanical systems just don't stop working, and any repair is far simpler than what is explained in that discussion, plus any mechanical repair can be done in the field not likely with electronic. This is a funny video but makes serious points:
View: https://www.youtube.com/watch?v=b9uiFejyTbo
Another issue:
View: https://www.youtube.com/watch?v=TrkH2bIKrK0
Of course there are advantages to elect shifting, it does shift precise when it's working as it should. Some say there is an advantage that it can shift many gears at the same time, big deal, SIS and Friction did that for years! Some will say minimal maintenance, that's a tie, well sort of, like I said before a battery cost more than a set of cables, but once the Di2 system is working it's trouble free just as the mechanical system is.
Look, I like things simple, simpler stuff breaks down less frequently because there is less to go wrong. I also don't like the idea of riding 50 miles from home and having a problem I can't fix on the side of the road, I hate the feeling of having to call my wife to come get me because of some problem. I'm an independent guy, I owned a business in the past and own another one now, I don't like relying on people to do my stuff for me when I should be able to do it myself, and that's the way I am with my wife, I don't treat her like she's my mommy.
Again let me repeat, this is only my opinion, others will chime in to refute all that I've said and that's fine, I'm not going to refute any of it and cause a big argument here, it's just their opinion, neither are right or wrong, just opinion.
By the way, I actually prefer Shimano 105 front derailleur and Ultegra rear with 105 brifters, 105 is very rugged and will last a very long time, Ultegra shifts the rear just a tiny bit better, but the rest can be all 105 and you won't tell the difference especially if you use DA cables.