Rangers vs Recon-Both are quite different. Marines do a lot more amphibious operations (as their name implies), but the Rangers are still very good. Overall, the Rangers are bigger and do more operations, but they're different and there really isn't a "better" group.
As for snipers, for the Marines at least, you need to have combat experience to qualify. I think you have to be an E3 or E4 to even qualify. You can qualify for Recon right out of Boot Camp, so I heard a lot of Marines go to Recon first, serve there and eventually go to Sniper school (in the Marines, they're called Scout Snipers) because a lot of snipers are deployed with Recon, plus you can use the skills from Recon in sniper school.
Army sniper school trains special forces snipers as well, from what I heard, but I think you can qualify out of Boot Camp. Difference is, Army snipers are snipers, Marine snipers are Scout Snipers, and do a lot of Recon as well as sniping, so their training is longer and more difficult, but over all, both require you to be excellent marksmen.
Correct me if I'm wrong military buffs, but I believe the Marines are issued .308 Remingtons, and the Army snipers are issued .50 cal Barretts.
Hope this helped!
**EDIT**
By eligible out of Boot Camp, I mean you can qualify for training in the Infantry followed by the specialized training. Before any sniper/special forces, you must complete basic Infantry at least.