My youngest was 3 when we split and we went immediately with shared parenting.
Both of us worked so he was used to being in daycare during the day, and have each of us as equal caregivers evenings and weekends.
We went with basicly a 2 day rotation so that he wasn't away from either of us for very long. 2 days is a long time for a 3-6 year old, above that and their sense of time and scale is different and they have more of a grasp of what "See you next week" means. Up until 10-12 I would personally say that 3-4 days is preferable, above 12 it should be up to the child.
There have been no problems with my youngest over this, he is 6 now and settled in at both houses. If there had not been shared parenting with this kind of schedule, he would never have felt at home and comfortable at one of our homes.
If you both have been involved in caregiving all along, then NOT having shared parenting will be a severe shock to the children. They are used to having 2 parents. No matter how much you visit, access/custody arrangements will leave them feeling that one of their parents is now gone. A shared situation gives them more, not less, 2 homes, 2 sets of toys, two parents.