Formidilosus
Like You I have been a slotted sniper for several years. And have seen/used all the scopes you mention, by far the most problems we have seen are with the MK 4 M2 3.5-10 SASS scope.
The M3 LR has been relatively trouble free comparatively.
I saw my first Nightforce failure the other day. One of the 3.5-15 F1 H58 reticle. Have to send it back to find out what happened.
To the OP I really don't think you can go wrong with either choice for your stated purpose. Personally I prefer Nightforce, but would be happy with with the Leupold.