40mm typically requires med. rings .... 50mm requires high rings.

There is a difference in alignment and cheek placement with the 40 being better, but if you shoot more " upright " then it is not as noticable.... But it can affect the shot and / or accuracy.

If you choose to go with the high scope mount with the 50mm scope, just be sure to practice plenty with it from several different positions and get used to it.