Size matters. As trivial as it sounds, it is the first important decision you have to make that can affect your sleep and your wallet. Having enough room to be comfortable and not disturbed is essential for deep undisruptive sleep. If you are a light sleeper and you easily wake up to any movements of you partner, consider larger bed where less motion will be transferred if you are further apart.
Bigger mattress will cost you more money. On average king or california king mattress will be at least 30% more expensive compared to the same mattress in queen size. That is a significant amount when considering premium product.
Don't forget to account for additional cost of accessories. If you decide to change the size of your mattress you will need to upgrade your pillows, sheets and mattress protector at minimum. You might think about getting a new bedframe, headboard or whole new bedroom set.