Question:
I'm assuming they both do the same job...but which one is better because difference in prices for these are very big?
Answer:
they do not do the same job.
a volume box goes in your effects loop to act as a master volume, or in front of the amp to have the same effect as your guitar's volume control.
an attenuator goes after your power amp, and before your speakers. allowing you to reach power tube distortion at lower volumes. if you were to put a volume pedal in after the power amp, you would blow up the pedal, and possibly the amp as well.
Answer:
actually they don't do the same job. The volume box is simply a pot placed in the effects loop of an amp. What this does is allow you regulate the signal coming out of the preamp before it gets to the power amp. The overdriven sound you'll hear from this arrangement is coming from the preamp tubes only, the power amp (which is were the sweetest tones come from) isn't working hard at all.
The attenuator, on the other hand, goes after the power amp but before the speaker. It acts as a load on the amp, usually dissapating the power as heat and only allowing some power to reach the speaker(s). The advantage is it lets you drive the whole amp and get that sweet compressed, overdriven power tube sound we all love while helping to reign in the overall volume.
Answer:
Think of the volume box as the same as your amp's master volume. Think of an attenuator as a third volume knob that goes after you crank the amp as hot as you want it.