Mainly steel frames. Cheaper and stronger than aluminium, easier to weld and weight is not a critical issue. In terms of buying one, it really depends on what features you want. The most basic ones have no way of altering the resistance apart from changing gear. More expensive models can have controllable resistance and tend to be quiter in use. My advice is to buy 2nd hand as a good percentage of people that buy a turbo trainer start off with good intentions before realising that riding a turbo is mind numbingly dull.