The best soil for each of them is the soil they are grown in at the nurseries. The nurseries know what they are doing and the roots have adapted to whatever mix they were grown in. I don't think either needs to be repotted at this time.
The Dieffenbachia likes bright indirect sunlight with little or no direct sun. Allow the top half-inch of soil to dry in between thorough waterings. Watch for spider mites. This plant inevitable grows very tall and with its heavy leaves becomes top heavy. The solution is not an ever larger pot; it is to prune it. Pruned stems readily put out new growth and tip cuttings easily root in water or damp soil. Some people get mild skin irritations from the sap. Keep it away from toddlers or pets that might be inclined to sample it!
Your Croton likes a couple of hours of direct sun every day. An east window is great. It tolerates cool temps more than warm temps. Water as soon as the surface of the soil feels dry as Crotons do not tolerate dryness very well. It, too, may become leggy over time and will do best if pruned periodically.
