While I don't need a power reading for cx races, it would be interesting to look at after the fact. But I use my cx bike for road training and gravel/roubaix type races and want to have power measurements for that. So which do you think would be the safer option; crank or hub based? Basically which electronics would stand up better to the possible abuse? Not too worried about the weight issue in either case.
Is budget an issue? The new Stages left crank arm based units look pretty awesome to me. http://www.stagescycling.com/
And they seem like they are in a good spot to keep them pretty well protected from debris and such. The reviews seem pretty favorable as well. But I think they currently have a decent wait time, as they are making and shipping pre-order and early adopter units first.
I have used SRM's, Quarq's, Powertap's and Stages. They all have worked fine for CX. Just chose the one that fits your needs and budget.