The general rule of thumb <since your not going to be level 20 for a great deal of time> is Wiz untill 200 and then Mana.
I am not sure how easy it is for druids but i found wisdom very easy to aquire for my Cleric, the trick is to ry and get wisdom with a little mana that way you get the best of both worlds.
for example instead of using:
Velium jasper ring. get hold of:
Gold polished Quartz ring. if you buy the gold bar and the poilshed quartz and ask a Jeweler to make them for you it would even be less Pp.