The sps varmint comes with a heavier contour barrel, while the other sps' come with a sporter weight barrel.
The heavy barrel is ideal for range/target/high volume varmint shooting due to the fact that it takes longer to heat up, so more rounds can be fired before you have to let it cool.
Theres differences in the stocks as well... but the heavy barrel is the main one and the reason people choose it over a regular sps for a range/target gun.
However if you're planning on hunting with the rifle the heavy barrel may not be what you're looking for... as its just more weight to carry.