The tide is turning. There's a reason that converting the smokeless MLII into 40 and 45 cals has become a cottage industry. This is leading to a much better offering of .40 offerings. Hawk, Lehigh, Hornady, Parker, and Barnes (off the top of my hat, I'm sure there are others) all now have .40 bullet offerings.
The bullistics of a 200gr/.40 at 2500 fps (which is easily attainable with BH) trumps a 300gr/45 all day long, and even more so over a 250gr/45. For WT, there's just no need for a 300gr/45. What you gain in terminal performance (though it isn't needed) is lost in drop, drift, and recoil.
While you can shoot a .40 saboted out of a 50 cal. I have yet to personally see this done with MOA or better accuracy. I know others have, I have not. Using that thick of a sabot typically creates problems.
If you want to walk into Gander Mountain and buy your components off the shelf and have no great desire to shoot tiny little groups to 150 and 200 yards (without even compensating for drop), then get a 50 cal.
If you really enjoy shooting, are willing to think ahead and order your components on line, and want a MLer that is hell on WTs well beyond 200 yards, get a 45 cal.