Will you turn turrrets? If the answer is no, use a 1" tube. 1" is cheaper and lighter in general and has more ring choices, but this is getting close. If you will turn turrets, somewhere between 400 and 1000 yards your scope will run out of adjustment depending on scope, load and zero. Also, scopes are more acccurate near the center of adjustment.
40 vs 50+ mm. . .Will you look through your scope before legal shooting light, after legal shooting light, or will you hunt a very dark woods? Distance also plays into this. At a longert distance, around dusk, it is nice to have the larger bell for a clearer image. For most people, the 40 is better for weight, which probably should be the driving factor. Not sure. I like a 13 - 15 lb plains setup, so the 50 bell was a natural for my gun just to weigh enough and balance well.