Tough choice....
Each band has, IMO, two absolute classics under their belt:
Bonded by Blood and Pleasures of the Flesh (Chemi-Kill!!!)
The Legacy and Practice What You Preach
And each band has also released some other strong, and some not-so-strong albums.
Tough call....but Testament was less one-dimensional, so that means something. But "Bonded By Blood" is fierce....when I first heard it back in 1986, my jaw dropped. Neither Metallica or Megadeth were this ferocious.
Wow, I still can't decide...