Depends...are you boiling 5 gallon batches or 10 (or maybe 15)? Are you using a converted keg, a stainless pot, an aluminum pot, or some other vessel? These will all factor in your decision. I have a converted keg and want the flexibility to do 10 gallon batches. I'd be waiting all day for a 75K BTU burner to get up to a boil. Mine is 170K BTUs. Basically, I view it as a tradeoff. Is it worth waiting X minutes less while using Y amount more of propane?
Unfortunately burners vary significantly with their BTU ratings. The burner type (ring vs jet), whether its high pressure or low pressure....it can really be a pain in the arse to select a burner. I would listen to others responses on this thread....pick one of those and just give it a shot.
Also, make sure the base is big enough to accomodate your kettle, esp. if you're using a ocnverted keg. My burner rocks (Bayou Classic, 175K BTU), but it's only 14" diameter, and my kettle is 15". With the lip around the base of the kettle, it won't sit on there, so I have to use the grill from a charcoaler. No biggie, but I wish I'd have bought the right thing in the first place!
Having used both a low output and a high output burner, I have to say that high output wins hands down. Yes, they are louder, but speed rules. Look for a high pressure set-up with a burner that looks like this:
The high pressure set-up will have a regulator with an adjustment knob on it.