Try out both and see which one you like.

If you like Death Metal then the Valvestate. Chuck Schuldiner used this amp and a lot of these death metal bands from up top(Finland, Norway and Sweden) used these.
I don't know too much about the tech21, so I'm kinda one sided.
Well, you can call me crazy
You can call me wrong, 'cause
See I was born a liar, albatross
Fly on, fly on
never seen let alone played the Tech 21. the Marshall is crap though. i have played those. spec wise the Tech 21 is interesting. i really doubt the marshall will give you anything your current amp doesn't.