king or queen?

So Nick and I have been casually shopping for a new mattress. We have had a sad history of not-so-quality mattresses in our time together. When we first got married, we were sleeping on Nick's double mattress from college. Then my parents upgraded to a king bed and gave us their (10 year old) queen mattress, which we've had for the last 3 years. We've tried to make it comfortable by adding two feather beds under our organic cotton sheets. But we still wake up with sore backs. We decided that this time around, when we are buying our own bed for the first time, we wanted to go with a natural mattress. We've shopped at a couple local natural mattress stores which further cemented our belief that we want to go with the most eco-friendly option possible. With all the chemicals, fire retardants, and other things that go into a typical mattress, we were pleased to see that the mattresses we tried out were chemical-free. 

We've settled on the exact mattress that we want to buy from European Sleep Works , except for one thing. We can't decide if we want to stay with our queen sized bed or upgrade to a (European) king. At the price of the mattress, it's not too much more to go with the king. If we get the king bed, we will have to buy all new bedding, rebuild our headboard, and it could end up looking really big in our average-sized bedroom. On the other hand, every time we've stayed in a hotel with a king bed, we've liked the size. Nick also pointed out that with our plan to eventually have more kids, our queen bed could get crowded pretty quickly... So what do you think??