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@Dammit
It's a bit difficult to call as it's not a direct comparison, but I've ridden Ultegra Di2 and Dura Ace Di2 back to back, the Ultegra with Shimano cranks and the DA with Cannondale Hollowgram/SRM cranks.
On the road they both performed flawlessly- you knew the shift had happened because the pedaling effort required changed, but it's pretty much silent, and so fast- the shift speed is on a par with rear shifting.
However, and this is where it's not quite direct, doing turbo drills I found that I had to back off slightly on the Dura Ace during the shift, which was a slight disapointment as I had read that you could shift whilst under load, pretty much irrespective of that load.
I wonder if this is because the Shimano chainrings are inherently stiffer than the Cannondale Mk V's, and if I'd has the Ultegra bike on the turbo and tried shifting at ~500 watts it would have performed better.
But I didn't have both bikes at that point, so I never tested it.
This never occurs on the road though- it just works.
My choice of cranks for a new bike would also bear in mind whether or not they are compatible with a power meter, and this is where the modular cranks such as the Rotor, Hollowgram and so on really score, as you can buy a Quarq, Power2max or SRM spider and bolt it in.
You can also change from 53/39 to 50/34 very easily, again by swapping the spider out.
I have Hollowgram cranks on the crosser and the road bike, the crosser has 50/34, the road bike 53/39, but I can swap that with a large allen key and a lockring tool.