I have friends on SOT and friends who secured the SEAL screener.
Neither is easy and most people self select before the SOT tryout/SEAL screener even begins.
SOT tryouts are intense and really early in the morning. You will bear crawl all over the place, need to be a good runner, be a good swimmer, and be mentally tough to not quit. You also get interviewed and may be rejected then. Google "devils mile workout". One day of tryouts a few years ago was back-to-back devil's miles (tryouts are a week long).
The screener is generally taken your 2/C (junior) year. If you're a real stud, I know a guy who got a spot as a 3/C (sophomore). He was medically dropped (symptoms of hypothermia/inability to adapt), spent a whole year working out and gaining a ton of weight (to stay warmer...think XC runner build to something more like a football safety), and secured this year as a 2/C. This year's screener went nearly continuously from 1500 Friday to 2300 Saturday. Most drops occur before the sun comes up - you go all night. The first thing you do is a PST. If you don't pass, you don't go on. You then spend the rest of the night doing pass/fail swim tests, getting in the Severn River, getting sandy, crawling around on Hospital Point, and doing a bunch of exercises with an unwieldy block of taped sand called a "sand baby". It is very hard.
A lot of people in SOT do well on the screener, but many wash out too. You can do extremely well on the screener without being in SOT. Some of the highest performers I have heard of were on the Triathlon Team and the Endurance Team. From your plebe year to your 2/C fall, when you'll likely take the screener for the first time, you can go from decent shape to screener god if you are truly committed. A former company mate of mine was middle of the pack PT during plebe summer, scored near the top on the EOD screener 2/C year, and just selected EOD. This person absolutely went beast mode for the better part of two years, though, and you will almost certainly not understand the level of commitment this takes until you are doing the work. A key to success is finding an upper-class to mentor you.
This is getting long, but one last thing. If you secure (finish) the screener, you are ranked against everyone else who secured. The top x number of people (determined by Big Navy/USNA summer training dept.) get an opportunity to go to SOAS over the summer, which is the actual tryout to become a SEAL. In a sense, the USNA screener is a screener for a screener for SEALs. More people wash out at SOAS. After SOAS, the USNA SEAL community will select x number out of the top performers from SOAS to become SEAL officers upon graduating. After you graduate, you go to BUD/S, where USNA people tend to do much better than people from other places, but some still wash out. If you pass BUD/S, you're finally a SEAL.
Being a SEAL is a great goal to have, but know that you're about to undertake the most difficult training of your life. Nothing will compare. All of this is done on top of other academy things like academics, which if neglected, will result in not being selected (your whole record is taken into account).
Source: Former roommate was just selected for SEALs, former company mate was just selected for EOD, 2/C in one of my classes just secured the SEAL screener.