I honestly don't think you can go wrong with either. I had a 2011 FLTRX with a 23" wheel and yes, I did cut and rake the neck on it with a Bagger Nation rake kit. Hands down, it handled better than the factory setup. The only thing you feel difference wise is the extra weight of the larger wheel. To me, that was a minor complaint if even really a complaint at all. At highway speeds, it was actually a huge plus from a stability standpoint. I do see a lot of people mentioning that you should just use trees and not cut it. I highly disagree. I've ridden a close friends street glide that has a 23" with just trees, (the pickard kit was what he used), and it is a completely different feel all together than my last bike. To me, it was no where close to having the same stable feel to it, and he agreed after riding mine.
Now to answer your original question, my personal opinion, and that's all it is....an opinion. I personally like the 26" wheel. I just feel it looks in proportion to the Road Glide better. The Street Glide is fine with either, but to me, the RG just screams for the bigger wheel. My current bike, a '12 Skunk CVO RG, is getting the 26" now, so I definitely practice what I preach. But the biggest thing is to use a good quality kit no matter what size you do. The benefits will far outway the few extra bucks.