If you can only get one of the two, start with a .338 lapua magnum. You will get more use out of it. I have both 50 and 338 so I'm pretty objective here.
338
- easy to get projectiles off the shelf
-standard large rifle magnum primers
-not as noisy
- very accurate
-15lb rifle
I load top quality component match bullets for $2.25 because I'm resourceful
50
- 50 cents a primer and can be a pain to find (stock up and you're fine)
-projectiles usually need to be ordered but are easy enough to get
-heavy rifle 25-35lbs
-pure awesome to shoot...once you find a good spot
-I load top quality component match bullets for $4.50 because I'm resourceful and buy in quantity
I'm not comparing performance because that has a lot more to do with you and how much you shoot than either cartridge
338
- easy to get projectiles off the shelf
-standard large rifle magnum primers
-not as noisy
- very accurate
-15lb rifle
I load top quality component match bullets for $2.25 because I'm resourceful
50
- 50 cents a primer and can be a pain to find (stock up and you're fine)
-projectiles usually need to be ordered but are easy enough to get
-heavy rifle 25-35lbs
-pure awesome to shoot...once you find a good spot
-I load top quality component match bullets for $4.50 because I'm resourceful and buy in quantity
I'm not comparing performance because that has a lot more to do with you and how much you shoot than either cartridge