pk=k it is a bayonet mount, you will need the lens and a pentax k to canon ef mount
m42 is a threaded mount, i believe the m42 version is a little bit less widely available, and slighlty more expensieve, but rice fluctuate and this was a few months ago. To use the m42 version, you will need an m42 to canon ef mount adapter. I haven't heard of one mount being superior to the other. I would say to go for the pentax k mount, as it should be easier to find other focal lengths to match your 50mm in this mount, however, if you have an interest in old soviet glass (helios 44-2, tair 11a, jupiter 9 etc.), you may want to go for the m42 version. Either way, the adapters are usually less than $30, even for a quality one