I would go with the Weaver T10 if you are looking to do another period type rifle. I have the MST100 but it was an impulse buy. I do like it but I don't know that I'd buy it again. If someone were willing to give me what I paid for it, I'd be very tempted to go to a Weaver as a replacement. The MST is nice, its very clear and the range adjustment so far is pretty much dead on matching the USMC dope chart to ammo so its pretty much idiot proof that way. But its expensive for what you get. That being said, maybe I should get a Weaver and compare usability before I shoot my mouth off! Would be a good excuse to do a cheapo earlier M40 type rifle, I'd probably go the same route with a 700 SPS again, see if I could find a wood stock, maybe trim the barrel down to 24".....
The MST-100 has the ease of use, after all it was made to be used by Crayon Eating Marines

The Weaver T10 are nice scopes. Not as clear as the MST-100 in my opinion and are more inclined toward the precision shooters at the time. It does have an adjustable parallax on the bell so you have to take that into account when shooting at various distances to make sure your parallax is correct.

Here is Carlos Hatchcock sitting with his M40A1 rifle sporting a Weaver T-10:

The other scope to use for a "period correct" build would be the Redfield 3-9 that was found on the M40 series of rifles. A couple of them originally found their way onto M40A1 rifles during the transitional period. See picture below.
