To be the best at what you do, my suggestion is, full spec rejuv/enhance. From there you can either spend the xtra points on shield or flex or split between the two, however you want. I am spliting between flex/shield and will top them off with SCed armor.
The hard part comes in determining how you spend your RSP,s. I am using mine to boost phys defence mostly. Will also pump points into determination(?), the one that shortens the duration of negative effect spells like mezz. Then there is the one that boost the efffectiveness of your healing spells. Lots of choices to make, so few points to spend. :)