I bought a 8' Ranger sailing dinghy sailed it all summer and then moved up to my Cal21.
This was a good way to go for me. It let me make mistakes without getting into too much trouble.
I was bored of it by the end of the summer though.
The 21 is a whole different world. The rig is more complicated. The wind feels like it is more in control. It's harder to control the heel because moving your butt has much less effect in the bigger boat.