|I think you're asking about 2 entirely different things.|
MB = megabyte
Mb = megabit
You wrote mb? 15 mbps is 15 Megabit per second (Mbit/s or Mb/s) which is a measure of speed/transfer rate.
But what about the 100 mb? Mb? or MB? MB is a measure of storage space, not speed.
EDIT: for speed, you need to have a time element involved. For example, 50 miles is distance, 50 miles per hour is speed.