If you got a k03s to begin with, i'd stick to it, otherwise go straight for a k04.
I will be eventually going k04 due to costs vs a k03s are not much more. It all depends what you are after as well, 230 or 260bhp, both turbo's don't suffer lag and the size is not much different, k04 is slightly bigger and has a 51mm inlet, k03 series has a 50mm inlet. Biggest downside on the k04-02x series is the exhuast manifold, off course if you go k04-001 route this is not an issue as k03 manifolds are less restrictive.
But I am not sure on the reliability or boost levels of a k04-001.
You could go crazy and get a stroker kit and have a 2.0L 20v engine with a hybrid or bigger turbo like gt28xx... But its all down to what you want and how deep your pockets are.