Di2 or mechanical? I've had some issues with Praxis rings and Di2 shifters, for reasons I don't understand at all, while I've never had any problems with Rotor rings. Personally, I'd go for Rotor.
I've run two sets of Praxis rings (double and compact) with cable and Di2 and not had any issues. I mean, Shimano shift better but I'm hardly road racing and stressing about downshifting at 400W out of a corner climb. Damn it, I'd already decided on another set of Praxis :P
Di2 or mechanical? I've had some issues with Praxis rings and Di2 shifters, for reasons I don't understand at all, while I've never had any problems with Rotor rings. Personally, I'd go for Rotor.