If that is for that 7mm M70, Duke, I would get the 40. I don't like the bulkiness and looks of the big 50mm lenses on a 1" tube. I've got a 50mm on a 3.5-10 Leupold, and though it's nice, it doesn't look or handle real good on my M77 Ruger.
The bigger lense requires higher mounts and is less forgiving on the chin with rifles that kick.
You will get a larger exit pupil on the higher settings with the 50mm but I have never found a 40mm objective to be lacking in brightness at last legal light. For me the trade off isn't worth it. YMMV, but I'm with the previous guys on this.
Edited by tahqua