Yeah i agree with all the above comments with one exception.
The only advantage of the 338wm over the 338.06 in that length barrel is the availability of factory rounds.
Forget that factor & the 338.06 will definitely work better in that length barrel.

The 338wm really needs at least 22 inches & preferably 24-25 inches for peak performance.
But, if what you're after is a hard hitting (& sounding) 338 cal rifle capable of whacking out 250 grain pills for sambar, without having to worry about reloading then the 338wm in 20 inch will deliver. Not my choice though. Better options available I reckon.