I have not owned or shot either .338Lapua or 50BMG rifles, although I have looked at them quite a few times at the lgs (would like to own them one day when I can afford it). The price difference between two rifles can be a factor. A Barrett 50BMG M82A1 is $8900 while the Barrett .338Lapua was around $4700 which is almost less than half of the 50.
In some applications the 50BMG can be overkill and ammo costs something like $5/rd depending on what you use. .338Lapua has better BC and a bit cheaper overall, while still having the same range(or more) as the 50.