I played radiation sonic for a while and felt very powerful as a second support, but had trouble carrying the load if I was the sole defender. Things like mobs being spread out or my anchor running away could lead to a dead teammate without feeling like there was much I could do. After grouping with a rad defender on my 50 and finding he had the same problems, even at that level, I started trying out other defenders as my support main (used strictly for grouping).
I tried a bunch of different "healer" supports that I felt could handle being the only support in a group and wound up enjoying thermal the best. Nature came close, but I don't particularly like that cone heal. I also liked dark, but felt it went too heavy on the control powers near the end of the power set. Thermal is an active healer that gives me two heals and a nice set of buffs/debuffs. Sonic I'm taking because of that awesome secondary that really helps in groups.
I plan to take almost everything out of the thermal tree (it all looks good) and only a few sonic powers (probably the three single target nukes and then howl to spread around -res to a group). But, when it comes to sets and regular/epic power pools I'm at a bit of a loss.