Which is better, and why?

B & M 250 with a Blower Shop rebuild. Less heat and the teflon strips on the rotors give a better seal.

I was told a wieand is usually better. The B&M has 4 mounting bolts the Wieand uses 6.

I assume you are comparing these 2 superchargers because you are buying used?
If you were comparing apples to apples (buying new), the comparison would be a Weiand 256 vs a B&M 250. Having owned both, I would lean towards the Weiand. The earlier models with the oil reservoir rear bearing plate would be the best (2000/2002). Both are 100% Holley owned companies.
Between your examples, if the motor was a 454" or smaller, I would again go Weiand 177. If the motor is larger than a 454", and no larger than a 502", I would go with with the B&M 250 (actually a 671 would be better for a 502"). A good general strategy is to run a large blower, and turn it slowly to acheive lower outlet temps and longer bearing and seal life.
If these are used units, I would have Darren Mayer Performance Engineering do a thorough inspection and rebuild. There are modifications for all three that will make them more efficient and reliable. www.dmpeinc.com The cream of this crop (250" blowers) is the Blower Shop 250 billet aluminum (vs cast) blower. A friend of mine recently bought one to replace a Weiand 256 on a Merc 575, and it is a very nice piece. I have a BS 871 (purchased from DMPE), and like it a lot. www.theblowershop.com
Regards,
Steve