Dear Fran, This is a really good question! To be honest, I don't believe they require balance. You thought ambition was the opposite of contentment, didn't you? I don't look at it that way. It's not like a teeter-totter. They don't have to balance out, because they're not on the same spectrum.
I'm content and ambitious. I'm content with what we have and what God has entrusted to us. On the other hand, I'm not content to sit around and do nothing just because we've been blessed. To me, that kind of attitude is not contentment, it's apathy. I do what I do because I'm passionate about helping people. You can have peace and contentment and at the same time be a real go-getter - the kind of person who still moves stuff around and makes things happen. They're not inconsistent concepts. I think problems are created when ambitious people are driven by a lack of contentment.
Let me ask you this: Do you think Jesus was content? Do you think He was ambitious? The answer to both questions is yes! You'd have to be pretty ambitious, I think, to take on the sins of the world. At the same time, do you know of anyone else who had more contentment than He did?
More than anything, I think it has to do with what's driving your ambition. If discontentment or a quest for "stuff" is the motivating factor, then maybe they are on the same spectrum. But I think that's a really bad way to live your life. You can get tons and tons of stuff, but no matter what you get or how much you get, you're still not going to really be happy until you find contentment!