I’ve just thought. I’m assuming that you have control of the source here (eg, a games console with the option to output either format). If you don’t, then it’s probably better to match the output to the source.
For example, AFAIK Sky HD broadcasts at 1080i (or at least, they used to, I don’t know whether that’s still the case or not). The Sky box gives you the option to output in either 720p or 1080i, so whilst 720p may be nominally ‘better’ the signal will have to be converted. In this case it be better to leave it as 1080i assuming you have a Full HD TV. If your TV is merely “HD Ready” it’s probably got a native resolution of 768px, which is a whole other can of worms.