What a silly question, I might go 'round your house and headbutt you in the face for asking such things. New Onslaught, worth your time? Elementary, my dear chipmonk - it's fucking MANDATORY. It's possibly their best, easily on par with (if not better than) The Force.
As for Carcass, Heartwork is grand but it's most certainly not their best. get Symphnies of Sickness and the almighty Necroticism: Descanting the Insalubrious. Then you can get Swansong and Reek of Putrefaction - the Wake Up And Smell The Carcass compilation is well worth your time too. Swansong is essentially Carcass cockrock, but it's great in it's own right.