I had this same question over the weekend and spent a good amount of time researching. Consensus seems to be if you shoot a lot of 4k or do a lot of transfers to/from PC, a higher speed could be worth it. I'd keep apps on the internal memory and media on the sd card.
I found this review/test to be the best and really helpful and informative, especially the part about perceptible differences during typical usage. http://www.custompcreview.com/reviews/best-microsd-memory-card-review-roundup-2015-update/22978/3/
That said, I had a gift card to burn so I picked up a Sandisk Extreme Plus 64gb yesterday. I don't know that I'd actually notice a difference from a slightly slower one, but figured it couldn't hurt and should also be more future proof. I've taken quite a few 4k videos since, done some transfers to PC and it's been pretty darn fast with no hiccups at all.