A 458 win mag is of course all you'll ever "need", as it's external ballistics (500 grains at 2150ft/s) match the proven big game 450 and 470 nitro express cartridges. However, all else being equal, buy the Lott as it can fire both cartridges. Also, because of the long throat in the SAAMI spec win mag chamber the bullet actually engages the rifling sooner in the Lott. Even a win mag fired in a Lott chamber has a shorter jump to the rifling than in a win mag chamber. Not a big issue unless you want to shoot hard cast lead which should prove to be more accurate when the driving band is just off the rifling - according to the old buffalo - sorry I mean bison hunters. Look, I love the win mag, I shoot them all the time in my 458 Lott.
If you hand load, either cartridge is a joy to own.