I really liked that with the K10U, I was finally able to share the phenomenal sound quality of the K10 with my friends, rather than having them take my word for it. They were convinced.
I think that, as a general matter, you should opt for the custom fit unless you have a specific reason to go with the universal, rather than just defaulting on the universal. I can definitely say that I preferred my custom fit to the universal fit in my case.
Others may want a universal fit so they do t have to deal with possible refits. I think that as long as you have a GOOD set if impressions, that worry is overblown, and the better fit of the custom will be better in the long run to dealing with tips, insertion depth, angle, and all of the hassle that comes along with a universal fit.