Which knob?

[PrimusPanda]

Which is better, volume/volume or balance/blend?

[line6man]

There are a million threads already debating the subject.

Neither is "better," it's a personal preference and both have their pros and cons.

[FunkMetalBass]

You need to run a TB search. VVT vs. VBT (I presume you meant vol/blend, not balance/blend) debates are all over the forums. Both have advantages and disadvantages.

Further Reading:
Why Volume/Volume?

[darkstorm]

Either is fine, however you could typically get a little more fine tuning with pup blending with seperate volumes. Since you have 1-10 for each and with blend/balance its 1-5 each side.