many people find 3 under more accurate at the shorter distances that beginners usually shoot at, say up to 20 yds. At more distance, with 3 under, you start to have to hold over more, and split becomes more advantageous.
Another difference, at least in my opinion, is that split gives more control over the arrow has you hold it on the string.
I shot 3 under and liked it a lot, until I had to drop my draw weight. Then I went back to split because at light weights I had to hold too far over with 3 under to shoot any distance.
Now I shoot mostly split, but still fall back to 3 under for short range stuff... 3D at 10 to 12 yds. On some of my bows I've started using two nock points, one at 1/2" for split and one at 7/8" for 3 under.
As far as string goes, folks who want every foot per second they can get are generally in favor of fast bows and fast flight string, but that string can break down older bows that aren't built for it. As a beginner, you're better off to go easy and not worry about fine points like that. Just gset a good bow and learn to shoot that bow. Once you've learned to shoot and developed form, you can start to change things. A year from now, you won't believe how much you've learned.
Someone above recommended finding an old longbow with no shelf... the kind you shoot off of the top of your fist and can shoot from either side. Wonderful idea. I love those old bows. They're slow, but not expensive to find and can teach you what you need to know about archery.
You're going to have a blast... just remember to stick to the basics at first and don't worry about all the fancy technicalities