I like both TA and storm very much, but I'd lean toward suggesting storm for a first journey to 50 on the evil side of the fence. Storm will have an easier time with EBs than a TA will and there are plenty of EBs along the way to 50 when you're a villain (especially in the 40+ game, during which time you can have both tornado and lightning storm to whittle away at EB hit points). Since you're more concerned with being underpowered than with being challenged, storm might be the way to go.
Another benefit from choosing rad/storm or dark/storm that might matter to you has to do with the potential to stack thunderclap with cosmic burst (or with stacking dark pit and thunderclap). Stacking stuns can be helpful in ways that having a stun and a hold (ie: for a rad/TA) would not.
I think Rad/TA would be cool, and you're right - don't think I've ever seen one around! However, I think you'd enjoy Dark/Storm or even Fire/Storm particularly for your first villain 50 
@Fail (Used to be @Tux) and @Tuxedo Infinitus Defiant/Freedom/Champion
Favourite Toons: Prosper [Ill/Rad] Controller :: Infinitus [Fire/Elec] Blaster :: Pocket Dynamo [Fire/Shield] Scr :: Fast [Cold/Sonic] Def ::
Inspire [Plant/Storm] Controller :: Quality - SS/Fire Brute :: Double Down [DP/Kin] Corr :: Pwnz [Fire/Cold] Corr :: Fail [Fire/Son] Corr
... Dark/Storm? I have a Storm/Dark defender and he's so awesome that I wouldn't mind doing his evil twin.
On the other hand, I've never seen a Rad/TA around and I've never played any of the sets to a decent level. Will I enjoy it? Or does it feel underpowered?
Chances are this will be my first 50 villain, so yea... I hope I pick the most fun and rewarding one.
@False Fiction - Virtue / Defiant
Current projects - [Glaciologist - Ill/Cold Troller] [Cloudshaper - Storm/Dark Def] [Harald Wartooth - Elec/Psi Domi]