I personally have a Getaway and a 16 that I sail frequently so I may be biased. The real question you have to ask yourself is, what are you planning to do with the boat? Is it for casual sailing with family and friends? Also, are you able to leave the boat on the beach? Will there be times where you will be sailing solo? Have you sailed a hobie before? These are important questions you will need to answer before you decide. The Getaway is small, light, and easy to manage by yourself. The boat takes about 5 min to rig. If you have the family in mind, the Getaway can be easily righted with 2 people or +180lbs. It will also accomodate 4 people easily (heavier winds perfered) with about 900 pounds onboard max.
The 21 is an impressive boat, especially if you can get it at a good price. It is big and fast! It is nice with a lot of great features (front tramp, coolers, wings, roller jib, mast bob) like the Getaway. You can also invite the whole neighborhood on it too. The only problem I would see would be its size and weight. I don't believe you will be able to right it by yourself and may be tough even with you and your children. The boom may be a minor concern, nothing like getting konked on the noggin if you're not paying attention.
The Getaway is a very easy boat to sail, period. The 21 is bigger, roomier, and more impressive, but may be more difficult to handle on and off the lake.
I hope I have helped in some way. One thing is for sure, you better hurry up and decide. Summer is almost over!!
let us know what you decide.