The Assassin and Sorcerer offer completely different play style choices (as do the Shadow/Sage Consular mirror classes). The Assassin is primarily designed for melee DPS, but can tank if spec'd properly (although how well in high level Flashpoint/Operation scenarios I am unsure). The Sorcerer offers you either ranged DPS or healing.
If group play is your thing then yes, I would say as a (good) healer you won't be wanting for invites. I would think there will be no shortage of Lightning spec'd Sorcerers, so if you went that route you'd probably have a much harder time getting invites to random groups. I'm not sure about Assassins of either flavour, but certainly I don't think groups will be wanting for DPS. Tanks on the other hand, like healers, will always be in demand (good ones anyway).