People are always looking for the best deals. Like the author of this post he wants something cheap realiable and will make his car faster. When buying a turbo you have to go with what works and what will give you the best reliable gain. You dont want to get the cheapest thing out there. Superchargers are inherantly cheaper then turbos because they have less parts and less engine management. The majority of money spent on a turbo is tuning and getting your car running right.
Tuning a turbo isnt all about fuel and air. Its also tuning your turbo to the power band in your car along with the gearing in your transmission. Keeping the shifts within the power band and not shifting out of it. When you shift you want to be at the end of the power band and going into the next gear at the start of it.
you also have to build you engine to except boost and make a reliable machine that will run like stock. BMW have awesome enginneering and you dont want to throw a piece of garbage on there that will take away from it.
Superchargers have it easy because they are engine driven and you are always boosting. You can also tune these but out of the box they are going to be sufficient to your power needs. Unless you are building up a dyno queen or you strive to just be better then everyone else. Forced Induction is a serious modification to do to a car and care should be takin in doing it. Not to scare you out of it I would encourage you to learn and explore FI. Im not an expert but i have some knowledge of it and i find it very interesting. If you want to turbo your car do it the right way and have fun, dont build something that will break down every five minutes. Have fun with it but know what your doing!