The m should always be capitalized because it stands for mega
the b should be capitalized depending on what unit it is.
b=bit and B=byte
for those who don't know, there are 8 bits in a byte, so yeah, the b does matter.
your internet speed from speedtest and what cable companies advertise uses bits, or b (most are familiar with Mb)
Downloading things measures in bytes, or B (most are familiar with MB)