Don't forget that Xycraft tanks also act as liquid transposers, and that they pull from adjacent inventories. This is really cool, but you would generally need power for this and a lot more effort, not to mention space.
I think that in order to belance this, valves would need to be very valuable or consume quite a bit of energy.
I would simply make a few tiers of them, and make them upgradable. The various functions (balancing between tanks, filling buckets, filling from adjacent water, etc.) could simply be accessible in higher tiers.
The lowest tier would be a simple valve like in Railcraft. Cheaper and cooler, but less space efficient.