[sammyg2]

A waste gate is a pressure relief valve on the exhaust right before the turbine.
A BOV is a pressure relief valve after the compressor and before the intake system.

A waste gate is more efficient because it reduces exhaust bacxk pressure when maximum boost is obtained.
BOVs relieve pressurized air, after it has already consumed enough energy to compress it in the first place.
So as far as just limiting boost, a waste gate does it better.
BUT:
The big advantage of a BOV is that it does two good things:
it relieves intake pressure when the throttle is snapped shut (like when you shift) and prevents surging in the compressor. this is good for the compressor and makes it last longer and reduces intake temperature a little.
It also allows quicker spool up times after shifts because the compressor doesn't stall, then have to spool back up.