What pedal should I get for my amp should I get an overdrive pedal or a treble booster. I play a lot of classic rock, some metal (but very little) and blues.

Overdrive Pedals:

Visual Sound Jekyll and Hyde
Mi Audio Crunch Box
BBE Green Screamer

Treble Boosters:

Homebrew GERMANIA Treble Booster (Dallas Rangemaster clone)
BBE Frequency Bost Vintage Treble Booster

I have and LOVE the boss OS-3 You can mix and blend between Dist. and Overdrive, and it is a sweet pedal. but from your list I'd take the Jekyll and Hyde because it is also an OD/DS pedal.
I have the same amp, and play the same type of music primarily. For low to moderate gain I use either a TS808 clone or a Rangemaster clone. I find that the Rangemaster tends to push the amp a little bit more, probably just because of the amount of gain available in the circuit. With the tubescreamer you can really come to close to Eagles/SRV territory though. As I'm sure you already know, Vox is not the amp for metal, so you would need a pretty good distortion pedal for that, like a Metal Muff. Hope this helps.
Haven't you heard?

It's a battle of words