I started out as a split shooter, and had a lot of pinch, consistency, and aiming issues (cross-eye-dominant). I switched to 3 under, and have no intentions of turning back! I was immediately greeted with no pinch, tighter groups, and a more natural aim. My sight picture is cleaner, and I notice that my bow is quieter with 3 under.
Generally, though, the decision is up to the archer, and what feels right to them. Both styles work well, 3 under is a tad less secure, in that, you have less contact with the nock. With practice and a proper setup, however, this drawback can be controlled. Go with what works, but do give both styles an honest trial before deciding. FWIW.