what are your cycling goals?? ride appropriately for that. my cycling goals are general fitness and enjoyment, most of my solo rides are <60 mins, though I do some that are over that when I have a partner to ride with.

If your goal is to become faster more competitive it's all about intensity.

Hilarious. Asking you permission to use it when I can on my next group ride? :D

Time.

As a practical matter, since I have a full time job and a full time family, I have to always go with time over distance.

amount of distance over a period of time.

Easy. On my slow, heavy bike, time. On my fast, light bike, distance.

False dichotomy - I suspect that the answer you are looking for is intensity, either as determined by HR or power. Having said that, intensity over a fixed period of time will get you stronger.

You still have to get in the miles to get good at the longer stuff. There are few super talents who seem to perform well with 60 minute workouts even for centuries, but for most folks, if you're maxing your workout length at 90 minutes with no longer rides during the week, you'll be suffering pretty good on a century road race, or even a metric century for the more average genetics person. You might go out fast and rock the first 2 hours, but hours 3 and 4+ will pay your lack of mileage back and more.