I always preferred the 10 length on the heavy lance, even with 10 less speed, and quite a bit less damage. Just because as cavalry, it's very nice to be able to reach out and touch the other lancer coming at you.
The current damage of the heavy lance is 23 pierce damage, which is terrible. And after they nerfed it, I started playing around with a lance (which does 28p, and my +3 heavy lance did only 26pierce after the nerf). And decided to give it a try.
I ended up trading my +3 heavy lance and +1 wisby gauntlets for someone's +3 lance. And it was the best trade I've ever made. I love the speed and damage of the lance. I basically try to avoid going head to head versus enemy cavalry, but I still will do it because I'm competitive as hell. Sometimes I win because my lance gets to full extension before the heavy lance, other times I can throw off their timing by changing speeds and/or direction.
And honestly, a good cavalry lancer knows that you help your team the most by supporting your infantry. Supporting your infantry usually means attacking enemy infantry when your infantry is engaging them. Or else keeping off enemy cavalry who are swarming your infantry. You can do both very easily with the lance (enemy cavalry will be trying to kill your infantry as you lance their faces). Even when you're running screens on the enemy cavalry, you're better off not engaging them as either person has a puncher's chance of winning the 1v1. If you go and die right away to another cavalry lancer, you're hurting your team a lot (considering you could take out 3-5 engaged enemy infantry pretty easily if you play your cards right).
The extra damage of the lance is nice versus infantry as well. I wasn't even 1 hitting people with 6 ps with a normal heavy lance in strat battles, even when aiming for their heads. And the extra speed is nice as well if you're going through an infantry line and want to be able to stab one or two more people than you'd have been able to get off with the heavy lance. Also it's saved me at least once being able to get my lance out quickly when my horse is stopped and I have a cavalry lancer bearing down on me.
In the end, 23 pierce is just too low to justify me using it over a lance. And I'm doing better now with my +3 lance, than I ever did with the +3 heavy lance before it was nerfed to shit. Even if they made the +3 heavy lance back to 29 pierce damage, I'd probably stick with the lance now that I'm used to it. In the end though, heavy lance versus lance, or even light lance, all have strengths and weaknesses. It's just a matter of how you want to play the class that really matters most for picking the weapon.