Re: 30 moa or 40 moa base for new build?
A lot of the answer centers around the quality of scope that you have. As a general rule, the more you spend on glass, the better the quality and performance. Keep in mind, that when I use the terms quality and performance, I am not talking about glass/lenses, but rather the durability and repeatability of the turrets.
Scopes that are not "top shelf" tend to have more issues when they get close to bottoming out. They are not always precise and may not always return you to your zero.
When zeroing your scope with 100 MOA of total travel, you cannot assume that your 200 yd zero will be right in the pre-set center of the scope with 50 MOA travel up or down. More likely than not, you may find you only have 45 MOA before bottoming out. Having a 40 MOA rail will force you to do precision work at the very bottom of your turrets adjustment ability. Is the quality of your scope up to that?
A good exercise would be to run a ballistics chart for your 300 Win mag load to see how much further out a 40 MOA rail will get you over a 30 MOA rail. I have no horse in this race and am just too lazy to do it myself. But I bet, the additional 10 MOA will only buy you another 150-200 yds of distance. Let me know if it turns out different.