It's hard to argue with elec/sd and fm/sd for both the fun factor and effectiveness. I've got one of both and while the elec shielder has all the AoE goodness one could ever want, fire isn't far behind with FSC combined with shield charge and possibly fireball from the epic. Plus Fire has a killer single target attack chain thats well documented here on the scrapper forums to be chock full of awesomesauce. There's a reason every other post seems to be asking about one of the two sets.
I've also got a couple willpower scrappers and brutes and it's hard to go wrong there as well, granted there is no shield charge, but it's still a strong secondary. With scrapper primaries, it's really hard to make a bad choice. Chose whatever you think you will have the most fun with, a good concept/costume matched with */* scrapper powers and your set. Good luck to you, and I feel sorry for your tank, you'll likely never play him again. 
I've been tanking quite a bit lately (I know I know). With double xp on the way, I thought I try a scrapper. I wanted to try the elec/shield for it potential AoE goodness as my shield/elec tank is fun. After looking through the posts, I've changed my mind a dozen times. It went from elec/sd to fire/sd to kat/regen to kat/wp to elec/wp back to elec/sd. I would like something that can deal the damage that doesn't cost a mint. Please help me choose...
I leave my double xp goodness in your capable hands.