Curly, I had the same decision to make. I bought a Kimber Montana in .300WSM and needed a scope. I love my Elite 4200s, but, unfotunately, they are longer and heaver that most scopes in their magnification size and class. Moreover, Elites generally have the shortest eye relief, which is a factor on not only such a lighter gun, but also on such a powerful cartridge. As such, I decided to get a leupold VX-III, but I had a hard time deciding between the 2.5x-8x-36mm and the 3.5x-10x-40mm.
Like you said, the weight difference is not significant, and neither is the length. However, I ended up getting the 3.5x-10x-40mm. For what it's worth, the true magnification of these Leupold models are not 2.8x-8x and 3.5x-10x. In fact, they are a bit lower. Again, not a big deal, but just something to consider.
Bottom line is that I am extremely happy with the 3.5x-10x-40mm model, although I don't think that I would have been disappointed with the smaller version either. You can't go wrong. Just pick one.
Zachary