Class vs Struct - It actually can matter

So, the second of my optimization realizations today involves the difference between structs and classes. For those who've been with this blog for awhile, you might remember a post I made half a year ago, here. In that post, I realized that a list of structs uses a lot less memory than a list of classes when each item is crazy small and there are a huge number of them.

Today, I've ran into the inverse. I made Point a struct back when I made that realization, thinking that it was small and therefore memory savings were worth the savings. However, when you stick an item into a dictionary as a key, it apparently makes a copy if it is a struct. It also makes a copy if you query the dictionary with a struct as the key. This meant I could rack up half a million or more point copies by scrolling for 15 seconds in my skill tree. This caused the memory manager to go crazy, and introduce the lag I was seeing. Switching it to a class helped a bunch.

When should you use struct vs when should you use class? Here's my current huristic:

If the item is large or you won't be having a crazy huge number of them - Use class, the possible optimization isn't worth it.
If a large number of the items will be stored in memory in a list - Try struct, as you'll save a pointer sized (4 or 8 bytes) amount for each item.
If you will be using it as a key (or value to a lesser extent) in a dictionary - Use a class, as each query on the dictionary and every access will make a copy.