I'm probably going to be getting myself a Steyr ProHunter here pretty soon, and I'm stuck on the caliber. It's either going to be 8x57 JS or 300 WinMag. My local gunshop has the 300 WinMag on the shelf right now for $679 I think, but I've always loved the 8mm round. What's the difference between 8mm Mauser and 8x57 JS? Any help would be great. Thanks in advance.

I love the 8mm as well but the 300 WinMag is a brute.

For example, a 300 WinMag will push a 220 grain projectile approx 300 FPS faster than an 8mm will.  For a 150 grain projectile the difference is about the same as well.

They are one in the same. The 8mm Mauser was adopted by the German military in the late 1800's and was loaded with a heavy 227 grain bullet of .318 diameter. In 1905 the round was improved to a higher velocity cartridge with a 150 grain spitzer bullet and the diameter was changed to .323. At this time they added the JS to set the two versions apart. At this time in history the early version has been out of the picture for so long that you almost never hear the JS mentioned. When you go down to the local gun shop and by a box of ammo for your 8mm Mauser/8X57 you are buying the JS version. I would be surprised if any US ammo makers made the early version. There may be enough of the early rifles in Europe left that they feel it is necessary to put the JS designation on your rifle.
I am not familiar with the ProHunter but I have a Steyr Mannlicher Luxus model full stock carbine in 7X57 with a Zeiss 4X scope on it and it is just absolutely a great rifle.
Good Luck,
Tuco

Please consider this another vote against the 300mag. The 8mm Mauser is much more pleasant to shoot.

Thanks for the info and the opinions. I'm going to go with the 8mm.

I've got a ProHunter in .30-06.  I chose this caliber because I have a bunch (3) of Garands and wanted to control the proliferation of calibers I need to stock.  They're nice rifles, whatever the caliber.  I'm sure you'll enjoy it.