zeta-
After a recent eye opening lesson strictly on form, I would say both. Consistentcy of anchor (FORM), alignment, back tension and practice on repeatable form will result in consistentcy of arrow placement. So form comfort should lead to better aiming. Aiming is for hitting targets while form is for properly releasing the arrow by using the correct "set-up" ie: form. But is there a difference between split and 3-under proper form?
If your anchor is comfortable to you and your form is consistent than that is YOUR anchor. Don't worry about other people's anchors as they aren't the ones shooting YOUR bow. I recently started shooting 3-under and my anchor is my thumb-knuckle closest to my hand on my rear jaw bone because that is where my hand goes when I use proper form. If you are using proper form, your skeletal and muscle structure will dictate where your hand will end up at full draw. Again, these are just my opinions and not to be taken as if I, in any way, know what the heck I am talking about! Just putting my perspective out there.
Best of Luck. Archery is a very individual thing and it's always interesting to me to hear other peoples techniques and ideas. Enjoy your shooting.