I have both:
2015 Specialized Roubaix Pro Race Disc with Shimano Di2 11 speed
2019 Specialized S-Works Tarmac w/SRAM eTap AXS 12 speed
I have 20k+ miles on the Di2. I only have a few rides on the Tarmac so far in addition to pre-purchase testing, but here are a few initial comparative thoughts. I’d be happy to share more later as I get more miles on the Tarmac:
Speed of shifting: +1 Shimano Di2. When I went from mechanical to the Di2 in 2015, it was nothing but amazing. Tap-tap-tap as you shift from gear to gear to gear at incredible speed. SRAM e-Tap had a well documented irregular shifting in their prior generation wireless e-Tap. The new design available in the 2019 is supposed to fix that (there’s a super technical YouTube video showing this to be true). So far it has been “mostly” a great experience, but not the same level of confidence/speed as Shimano Di2.
Intuitiveness and ease of shifting: +1 SRAM eTap. Left side moves up the cassette. Right side moves down. Both together move the chain rings. Incredibly simple. I have been so accustomed to the Di2, that I forget the complexity. Perhaps it doesn’t matter when you get used to either. But where it makes a difference, is that you can’t make a mistake with SRAM (pushing a button vs a lever). Also, in cold weather it is easy to find the lever to shift with SRAM. Much more difficult to click buttons with frozen fingers on Shimano.
AXS 12 speed: The new drive train design - 13 tooth separation in the chain rings and 12 cogs in the cassette - will be a game changer in performance cycling! If you want to understand further, there’s a Forum thread on this topic and a great GCN video (both below). If you want some specific experiences I have had thus far to highlight this, LMK.
