Nebularium is a great album, it is more progressive i would say than the others. Also Speed Strid isn't on it, their first vocalist Benny is on it, he layed down some vocals for D-Generation too though he wasn't in the band then.
As for the Testament thing, i can see how you think they are a poor mans Metallica but after you have heard every Metallica song a few hundred times you want something different and Testament fits that perfectly, it's heavier, faster, in terms of solos guitar work is better but that is just my opinion.