well, it all depends on what you plan on doing to your motor.
if you stying with the stock bore and are planning on adding a pipe and reeds maybe then get a 30 mm
if your going full out 240kit stroker etc then i would look into a bigger carb(35,36 mm)
as for brands some like mikuini some like keihin i have heard kehins areeasier to tune but also are a little more expensive.
if your on a budget look at keihin pj series