It depends on several factors. First, no matter which order, your emphasis with your children should be to get outstanding grades to secure scholarships. Possibly have them get Summer work to contribute to their fund, even if it's just for books.
If you were a client I would ask you what rate you're paying on your mortgage and how fast you can pay it off. Usually, when you have a solid goal you have more incentive to throw money at it.
Don't break the bank or underfund your retirement to put kids through college. I managed to self-fund through PhD. and pay back the loans. Help them choose good majors!