That would be my recommendation. It takes as little space as a chair. I have a dark gray one that blends nicely in the office environment, and I didn't need to change anything -- I even kept my chair for visitors.
Does sitting on a ball helps your health? I don't know of any studies, but speaking for myself, when I sit on a regular chair for more than a couple of hours I get low back pain. With a ball I can work 10-12 hour days without any pain. (My theory is that's because the pressure point is constantly moving a little.) I'm constantly making little movements so I don't feel stuck. It works the abs for sure. It's fun -- when I'm thinking about a problem I can roll on my back and be lying on it and it definitely helps. I also bounce on it when I'm thinking or do balancing poses. I can do situps to shake up a little. When I'm tired I can lean on it at 45 degrees angle and it feels restful. It helps straighten my posture.
The downside is the posture -- I tend to slouch if I can't lean on something. To prevent that, I wear shoulder support straps when I work. That's why it improves my posture, otherwise would degrade it.
Another upside -- I can carry my "chair" with me anywhere (e.g. when I'm going overseas for a month). In that sense it's the opposite of a standing desk. I used a standing desk for about four years, but started having back pains with those as well. Similar reason I guess -- not much change in the pressure point.
I've been sitting on the ball for about 7 months now. It took me about a month or two to break in completely, during which I'd sometimes switch to the regular chair. Now it feels great and my body always prefers the ball.
You need the right size ball; they come in 75, 65, and 55cm diameter. I'm 6'0 and I use the 75cm. They are probably all similar in quality, but this is the one I use and can recommend: http://www.amazon.com/GoFit-75.... I have one for the office and one for home. For shoulder support I use this: http://www.amazon.com/dp/B00CM..., size Large (even though I wear mostly Medium for regular clothing). Hope it helps.