Does my contribution vary depending on my business structure? Is it better to be a sole proprietor or an S-Corp?

In theory it does not matter. When you are a sole proprietor, the defined benefit plan calculation is a more complex calculation that is based on your business income.

But for an S-Corp or C-Corp, it is just based on your W2. With a corporation, you have a little bit more control of your contribution because you can increase or decrease your compensation (assuming you meet reasonable compensation). It is easier for us to determine your annual contribution under an S-Corp or C-Corp structure. But at the end of the day, your business structure is between you and your accountant.