Forgings and billet have better grain structure which improves strength pretty substantially. A machined casting is certainly better than a net casting. However castings tend to warp a bit when they cool which may induce a bit of internal stress. When they're machined it could cause them to move a bit as material is removed. Neither factor is probably significant enough to really consider in a low round count hunting application - but as much thought as they put into building a great action I was a bit surprised they built it from a casting. High end bows are machined from billet, forgings, or extrusion. More cost effective models are cast, then machined. Granted, totally different application, but I have a lot of experience with the latter.
The price is certainly right for the Montana M1999 which makes it attractive (and probably explains why they built it from a casting) - It's probably cheaper or about the same price as buying a donor 700 Rem and having it blueprinted and almost certainly you would have a much higher quality end product with the Montana.
Good luck on the Unit 55 NM VV tag. I've put in several times for that but have yet to draw one!