The real difference is how they're used. In the classic Sonic games, the speed boosters were generally used as a level gimmick, and not necessarily in the forward direction, either. In the more recent games they're used to force you forward through sections they want you to be moving quickly for (which is basically everywhere). This is a big difference in design, using speed boosters for quick sections where they don't want you to need to spindash as opposed to all over the level.