The m420 is the mini version of the i420. The i420 will produce a better hit, but cannot fit in your pocket like the m420 can. The m420 has WAY more attachments available than the i420 does. The i420 is limited to the expansion chamber, where as the m420 has the quickdraw trigger, expansion chamber, sherlock attachment, oil attachments, the whole 9 yards pretty much. Basically, if you will only be using the piece at home, or constantly have a backpack or something to carry things around in, the i420 would most likely be more enjoyable, but if you want something that you can use both at home and on the go (can fit in your pocket with the original tube and can be upgraded with different chambers for use at home) then the m420 is for you