How do BBK underdrive pulleys compare to MAC, Steeda, March or ASP? I can get a set of brand new BBK pulleys for $35.00, and if they are comparable in quality to the other brands, I think I'll jump on that.
Aluminum ones are a bit lighter and for some people the few pounds matters. I got a set of new FRPP Aluminum U/D Pulleys for $70 off ebay and I can feel the car pull a bit more now that I got them in.
Pullies are pullies. The only real differences are finish. Anodized, billet clear coat or powder coated finishes. The powder coated finishes are garbage and wear off fast.
Like what was mentioned above, the aluminum vs. the steel pullies. Believe it or not there is a difference in weight for such small pieces of metal. I replaced mine with aluminum and held the stock ones in one hand with the aluminum in the other. There was a noticeable difference in weight. I wouldn't worry much about the weight factor though. It's marginal as to what it will do for weight reduction.