From my experience, I would say that Arista EDU's reciprocity failure is much worse than Fuji Acros. According to the data sheets, Acros is good out to two minutes without compensation whereas Arista EDU needs compensation starting at one second. I've had shots using Arista EDU that metered at ten seconds, but with compensation actually needed 80 seconds of exposure.

My reason for shooting Arista EDU at 50 is for better shadow detail. Like rthomas above, I arrived at this EI by making adjustments in my process until I was happy. I still continue to tweak things a little bit here and there as I go along.