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The two obvious choices are between Mu and Soul and each one will change the way you play your character. Mu offers you a lot of soft control though End Drain. EMP + Short Circuit will keep your targets without end for the duration of a fight. Conserve Power and Power Sink will keep you with endurance for the duration of said fight. Currently with my Elec/ Rad I have Mu and I am reluctant to switch to Soul.
The benifit of Soul is Power Boost. To me that's the best it has to offer both Electric Blast and Radiation Emission. Many players like Soul Drain and it can add to the overall DPS of your character. It provides a To Hit and a Damage buff for a few seconds. The recharge vs the duration (of the buff) vs the percentage of the Buff doesn't appeal to me. However, stacking Power Boost + AIM + Soul Drain can turn Short Circuit into quite the damage dealer if you slot it right.
It's a hard decision, but in the end you have to consider what your play style and what you want to accomplish with the character. The other Patrons offer other benifits, but I chose Mu and Soul because of their overall effectiveness to my character.
-The Elite-[50's]Va'Leria�X'hian�Stormy Monday�Radical Burn�Mo'Mentum�Heat-Source�Professor Blaze
-World Wide Evil, Inc-[50s]Soulfire�Perma.Frost�Kold Soul�Foxphyre�Pitch-Black�Corrupt Fusion�Cassanova Brown�Tyler Thorn�Iron Siren�Solaura�Fortunauta Wade�Look'Alike�Arctic Engineer
Thanks Valeria, for your advice!
I decided to go with Mu, since as I said, it fits the concept best. 
My main toons on Union:
Majer; Dark/Dark Corruptor
Caitlin Calathea; Plant/Thorns Dominator
Davallia Fejeensis; Empathy/Archery Defender
My arc:
ID 98789, What once was, will be again.
I have a Mu... Elec/Rad built as a Mu Guardian concept, and I like the way Mu Mastery plays into the entire build. Again, it's a concept toon and and I'm not min/maxing for any type of solo or PvP play, but for duoing and team play he works just fine.
Very nice to hear tell of a few more Elec/Rad toons out there, btw... Elec/ has been panned a lot as a Corruptor set. I don't know... I like mine just fine.
@Trystan Laryssa
I could use some help in choosing the Patron power pool for my elec/rad corr. What do you guys think would be the best pool and why?
Concept-wise, Mu Mastery would suit the toon in question best, but is it a waste to have so many -end attacks?
My main toons on Union:
Majer; Dark/Dark Corruptor
Caitlin Calathea; Plant/Thorns Dominator
Davallia Fejeensis; Empathy/Archery Defender
My arc:
ID 98789, What once was, will be again.