What will be faster when moving to sd-ext via A2SD+ Data or Dalvik-cache? (or can I move both with no extra-penalty)
As Matthew already stated, there's no unique answer possible to your question -- all depends on your usage patterns. Generally spoken, I'd say moving app data should be preferred: If you move the Dalvik Cache, it would affect all your apps -- while moving the app data only affects apps which store some data (and by "some" I mean "data size exceeds their Dalvik-Cache size"), so a good guess is it affects only a few apps. But this can very well be those apps you are using most, and which heavily access their data -- so it may or may not be the better choice in some cases.

Guess you've got to try. I'd suggest starting with the app data. A class 10 card should provide sufficient speed -- so if the results are acceptable to you, stay with this. Otherwise, go back and try with the Dalvik Cache. If this isn't acceptable either, the only choice left is buying a new device with bigger internal storage...