The correct answer highly depends on your goals. If you don't have any goals, and are just riding to ride, then you should ask yourself why you're bothering to worry which is better.

But in general:

- For long rides, or rides that are significantly longer than your maximum distance ridden now, be it 15, or 250 miles, you have to log the distance. Even if you're going at near walking pace, you absolutely must log long rides aimed at your target distance. So for a century, your long ride may be between 75 miles or 125+ miles. You can do it on less, but if your performance will suffer big time on race day when you hit those long distances and you're unaccustomed to distance riding. Forget speed on these rides - keep up as steady and consistent an effort as you can for that long distance, even if it's slow.

- For short races, like a 40k or shorter TT, or criteriums with lots of high power accelerations, it doesn't matter how long you can go if you can't put up the power (speed). You should be avoiding megadistance rides that tax your recovery for days, and focusing on shorter distance appropriate to race distance with appropriate high-intensity interval work. Yes, focus on power/speed on those intervals. These workouts will be a lot shorter than the long distance ones above, but they'll hurt a lot more in that short period of time - most people give up on these workouts after 2-3 weeks unless they've got a goal race.

The specificity of training is huge. In Socal, there are a lot of local triathletes (yup, not roadies) who are training for Ironman triathlons, and do 125+ mile hilly rides on weekends. I've joined them more than a few times, and invariably, I (not an Ironman competitor) feel like the first 80% of the ride is too slow, and I'll kill everyone. Then after mile 90, and on that next 2000 foot climb, I get dropped by all those slowsters. There's a reason why they're out there forever, because they'll be racing that distance.

In contrast, if any of these folks show up at the roadie AM interval short hillclimb, which is about 25-30 miles total, with several loops of a short hilly course but done a la criterium style, these same folks who drop me at mile 90 can't even keep up on the first interval, and usually get summarily dropped by the pack after #1, whereas I'll often be right up front the entire way. Those IM guys/gals don't do intervals because they don't need that for race day, and it shows.

And as an aside, as much ridicule as 'average speed' gets around here, it's can be a useful metric. The longer your ride, the more the average speed becomes a true average, as the overall elevation changes tend to average out more. On my 75+ mile ride, average speed is dead-on for my effort. On those hill sprints <30miles, not so much, as the elevation variation has much more of an impact since there's less averaging of flats & hills.

On an indoor trainer like the KK, average speed = best data point, as in that controlled environment, your average speed on a KK directly correlates to your average power.

Figure out what you want to improve - acceleration/climbing power vs long distance endurance, and that will help you focus on which one you can emphasize, even if you're mixing both in.