Which step next: $1/$2 6-max or $2/$4 10-max?

[QuickLearner]

There have been a few posters lately that have mentioned regretting moving up rather than learning short-handed play at the $1/$2 level. I've read all the "moving up" threads I could find and was ready to just go to $2/$4 but the comments from posters make me want to ask for advice. I'm ready to move so what should my next step be?

I have over 25K hands at $1/$2 and am winning. Not KILLING, but winning. My goal is to improve. I actually think I am starting to go stale at my present level. I have a sufficient bankroll; what I don't have is a strategy for pushing myself to that next level of competence. My stats are in order, so I need to step off the curb.

So, which step would be better for my long-term improvement?