Personally, of those 2 choices, assuming both bikes fit and are in top shape, I'd opt for the Cannondale Black Lightning - the improvements in road bikes have really come in the area of easier shifting, there are good 30-40 year old frames that are more than up to the task. And the C'dale Black Lightning is a somewhat legendary bike from it's era, whereas that Trek is a more modern, but middle of the road model. Disk brakes are better for off-road and bad weather use, but offer little benefit in dry, paved conditions.

Much more of an issue will be you getting enough training in in just 12 weeks to get yourself up to the task.