Something to put them in. Size and model. Now that you have decided on the reserve and main canopies now you have to put them in something. Y quickly degenerate into a personal preferences argument. Pull the handles, the parachutes come out.
Function is a given.
However there are differences in how the containers fit . If possible, demo jump several containers. Be aware that a major factor in fit is the harness. A harness that is either too large or too small will not be comfortable and may actually be dangerous. People have fallen out of harnesses that are too large and have had problems finding handles on harnesses that are too small.