it's really hard to say, Krzysiek seems more "normal" or regular dimunitive (just like Jarosław>Jarek, Zbigniew>Zbyszek), but unfortunally it has something unattainable to it that makes it (unlike Jarek, Zbyszek etc.) a little odd, that's why many people (including me) don't like the sound of "Krzysiek", so they prefer to use "Krzysiu" instead (it's indeed a Vocative form of "Krzysio", but such diminutives are often used in colloquial, spoken language in Vocative case with the function of the Nominative).
And I agree with tulipan:
it sounds more warmly
in general women
Krzysiu sounds much more childish (it's totally OK to call kids with this form) and tough guys won't say it, they don't want to sound like they were displaying too much affection, in my life I've had a very few male friends/family members that were using "Krzysiu" while among women, even just friends (not girlfriends), it's much more popular.