The answer is: it all depends. 720p has better motion; 1080i has better resolution.
You should always deliver with the same resolution as the source material. Converting from one to the other won't give you any benefit if you've already shot the material.
The truth is that, in the end, 95% of people don't care or can't tell the difference. So in reality, choose your HD camera on other factors, like lens quality, comfort in operation, or integration with your current workflow.