i can't say about either for low b. However - i chose the cube, and i do play heavy metal. It came down to:
It had the minimum models that i'd actually use, i had no problem dialing in sounds fast, and the effects were sufficient.
The vox is a great choice too. All the vox/marshall models were just a waste of space for me, and i didn't feel like tweeking effects. But if you want all ther vox/marshall stuff and a little mroe control, its your amp.
I dig the cube. Been wailing on some wicked recto setting lately with a touch of delay. I just like the cubes basic distortion flavors and the eq, really. Get's me what i need no fuss.