Some of them are really cheap, e.g. the ST Nucleo boards are about £6.
The only reason I can see anyone would choose Arduino over mBed is if you really need native 5V. One could also argue that while the Arduino "IDE" is totally awful in almost every way, at least they have one. mBed currently only has an online IDE which is powerful but slow and online, or Yotta which is nicely written but currently command line only, and only supports a few boards.
There are also a much wider variety of boards available:
https://developer.mbed.org/platforms/
Some of them are really cheap, e.g. the ST Nucleo boards are about £6.
The only reason I can see anyone would choose Arduino over mBed is if you really need native 5V. One could also argue that while the Arduino "IDE" is totally awful in almost every way, at least they have one. mBed currently only has an online IDE which is powerful but slow and online, or Yotta which is nicely written but currently command line only, and only supports a few boards.