Sentinel is probably more fun overall than Engineer to me. I don't think the class specializations make much of a difference in the game play in all honesty, although Bastion is better than Medic just based on power. That said I did play a Medic Sentinel w/ Assault Rifle training, and it is a fun character.
I agree and disagree with Red about the Engineer. Tech powers on the whole are weaker than Biotics (except on mainline missions which are Geth heavy). And the lack of innate defense skill is pretty big. Probably they should have allowed Engineer medium armor, that would have made things interesting.
But anyway, you do have a tech talent for everything, just need to know the application.
Sabotage is the bread and butter power against most token organic mooks, elite organics (except Krogan) and snipers (if you know what you are doing and know the duration... also gets them to weapon switch sometimes). Using it against Krogan is indeed a recipe for disaster early game, unless you have Kaidan or Liara with you and they are on cooldown. Or you have the next power...
Neural Shock is actually quite good against Krogan and pretty much right up there with Lift. You just need to concentrate fire on them on the ground. For missions with multiple Krogan at once this may be problematic, but you can save them for later when you have Liara, or take Kaidan if early. But there really aren't many situations like that, one of them being Virmire, the other being Feros. The main downside is you have to invest 5 in First Aid to unlock it.
Damping is overall not as strong with one exception. It does stun a target briefly, and that can be helpful on some of the husk-rush missions. But, it does increase the radius of your other powers, which is actually pretty nice.
Electronics / Overload is good for investment and a good power. Overload is powerful vs shields, and increases damage taken, an effect that should not be forgotten.
Hacking is useful on mainline missions and the Armstrong Nebula mini-arc. Oh and Luna, where the Engineer with Tali and Garrus will breeze through.
As for the specialization, I would favor Operative probably, but who knows? Medic is ok if you want to run around with more Neural Shock I guess. The medicine bonuses aren't very attractive to me.