M Sand stands for Manufactured Sand, Crush sand is also a manufactured sand.
M Sand is specially manufactured sand from a 10 mm size aggregates with a technology for removing dust either by wet or dry collecting method.
M Sand and Crush Sand are almost identical to each other except the finer particle size percentage and the shape of the particle.
M Sand do have more cubical or round shape and has less amount of fines passing through 150 micron, this helps in making it usable for doing plastering and waterproofing.
Crush sand is used for doing concrete from a very long time and is good for concrete production if the mixes are designed properly and the source of sand is having consistent production to same properties.
Normally crush sand is the first choice by almost everyone due to the cheaper price over M Sand.
How to differentiate between
M Sand looks like river sand, the more gravel type cubical or roundish particles and less amount of dust in it which is also look blackish.
Crush Sand can vary depending on the type of manufacturing unit such as jaw crusher or VSI or any other technology. but crush sand is easy to identify as its just a crushed rock sand.
You should check the all properties of crush sand such as gradation, water absorption, specific gravity etc. before doing mix design and should only be used after doing trials at laboratory.