The OP question is quite interesting one.
On the face of it, two new environments and various car mods make it a bargain. Even looking at last year, Argentina Turismo employed mods in a brilliant fashion with LMP2 on showcase, but next to that having amazing aston martin safety car, giant Michelin clock and awesome rescue cars... that's only possible in LFS S3.
But...if you want to race...with lfs s2 you will be able to race in most of RTFR races, 90% of FM fox and Fm gti races, and 90% of mrc fbm races. Along with others, but only these 4 will give you something to race FOUR times a week. Getting S3 there wont change much. Hell, most of servers are still on LFS 0.7A, so you can even use lazy addon.
If you wanna check and drive new cars, beware that 95% of mods are crap. Some are good, but none are better than original lfs cars.
And if you want to drive new tracks from s3... Rockingham is buggy mas and worst track in LFS. Layout square is just big flat piece of asphalt and concrete... layouts can be made on it, but layout editor is so basic that they are not good, definitely not better than lfs s2 tracks.
So yeah, it depends on your wishes and abilities. If all you want is big field racing few times per week and are content with training on other days s2 is ok. On the face of it, lfs s3 gives a lot, but when you dig deeper, there's not much good in it.