And here I thought I did not have a twin
Understand you perfectly well. Very similar story here. Though, I did push myself and got my 4 lvl 50s. Oh, and I keep rolling my favorite class even though I know the story by heart and delete those toons more often then others.
I do not play any of my 50s (beside getting HK47 once) but they are still useful for the craft. If you do like yours - just run him from time to time through the low levels and collect materials.
As for your choice now, I do not really understand your troubles with assassin. Tank-o-sin is a god. At least equally immortal. Even before you get yourself a doc. After that the game become a joke. Tanks safety, rogue DPS and invisibility and even a small self heal on top of it. Plus, you want to finish the story, right?
Though, have to agree, marauder is simply very cool. Not strongest or anything, but cool!