Most people do not know the difference. The former is Megabits per second, the latter is Megabytes per second. 1Mbps is 8 times slower than 1MBps. For quite some time now, the byte has been the standard for measurement in computing. It's quite deceptive to consumers to advertise with a non-standard form of measurement that most people don't even know there is a difference. It gets confusing to me sometimes. If I need to download a 17MB file on a 15Mbps line, how long will that take? It's much easier to figure out if you know you're on a 2MBps line (rounded up).
Computer companies don't sell hard drives or CDs or RAM in the Megabit scale to make it seem like there is more than there is, so why do the ISPs do that? It's deceptive, it'd confusing, let's get rid of the madness.