I have Kolstein's and Oak as well, its funny, I was really liking
the Oak, then one week my teacher starts on this tirade about
how our local luthier convinced her to buy some and she just
HATES it
In spite of her I still think the Oak is more delicate than Kolstein
and find myself using it more. I've tried Pop's and like Ken thought
it was too messy. Also have a cake of Nyman's that I've found
usable.
I don't find any rosin to be so spectacularly better than anything
else that I would get religious about it.