It's not as much the duration itself, as how much you can achieve during that duration. With a 60 minute meditation, it's possible to reach far deeper states of concentration than with a 15 minute meditation. If you can accomplish that, then 60 minutes is probably better. On the other hand, if your concentration skills aren't developed enough yet, it may not be possible for you to meditate that long effectively, or you might put less effort into it since you feel you have plenty of time. I still sometimes get the best results when I'm short on time, because I'm forced to actually concentrate and don't have high expectations.

I'd say to do 15-minute sits at first, and then gradually push up the duration when you start getting the feeling that you could go on for a while longer. Experiment with what works best for you.