I have never went sf, but nearly every post has a few sf just for recruiting. If you are wondering I would find out if your post has them and request information from them. It is very rigorous both physically and mentally, and getting a scope of it beforehand should help you make a decision.
You could also ranger, which is a privelage for the army and an extreme privelage for the marines and other branches, like for example a marine needs to be airborne qualified and be an e6 to go through army ranger school. Ranger is less sf and more elite infantry, and ranger school is the best military leadership school around.
Either choice is severely difficult, with their own rewards. It is up to you to seek out recruiters to find out what you want, as things like requirements change constantly depending on the load requirements of each. I got begged to become a ranger after ait, when I went from running a 35 minute two mile being a fat drunk to running a 12 minute two mile still being a drunk. I desided against it, they thought I had what it took and they were in a shortage that year, they tried to push my brother to do the same. I decided more school was a no go and entered regular army service at fort hood, which now I regret not going the ranger route.