The biggest advantage to the 40X is that you can get it in a single shot. You can also get a repeater if you are shooting across-the-course matches. The repeater is clip slotted for stripper clips. The 40X stock is pretty nice. You get a heavy stainless match barrel on the 40X, which is better than the regular Remington barrels, but not as good as a top quality custom barrel. THEY ARE NOT TRUED OR LAPPED. I have blueprinted a lot of Remington actions and some of the crookedest ones have been 40Xs.
The single shot action is the only real advantage. You still need to have the action blueprinted, a match trigger, and maybe a new barrel.
Unless you just have to have a single shot, start with the ADL and save a lot of money. Or, find a single shot XP100 and convert it into a rifle.
Jack