70 - cigars will smoke hotter, burn will be wonky, and it will taste more bitter
65 - cigars will smoke better, burn better, and taste better.
Can't really elaborate much more than that. 70% used to be "conventional wisdom." I assume the thought was "higher rH = more retention of oils" and I suppose that's possible. That being said, I've never had a cigar taste better at 70% than it did at 65%, and occasionally going down to 62% is even better.
Try for yourself - grab a 65%, a 70%, and a two tupperware. Put identical sticks in each, and give it a month or two, then smoke each one.
I do know of some BOTLs who will age long term at 70%, and smoke at 65%. I don't have the space or resources to do this but I've yet to see any negative results, and I know many other BOTLs who have aged at 65% for years and had fantastic results.