I am with you on the 50 mm. I went with a Vx111 6X20X40 and had the custom shop install the M1 Knobs. I found it to be the best compromise. You can order it directly through Leupold. You paythe same due to there set pricing! WHen its there you can have them install any recitcal you want.
I think the MK4 are stronger built, but I have not had an issue with mine.