For me, Pantheon is all about getting your enemy to that 15% hp level as quickly as possible. An early BF sword turns Heartseeker into a hero shredder, especially if you have some armor penetration. Aegis -> Heartseeker often leaves my opponent around half health, if not worse. From there it's chasing with spear harass for the kill. If you run with exhaust you're essentially guaranteed a 1v1 kill unless your opponent has an escape mechanism.
Heartseeker is also the best method for dealing damage while exhausted. I try to save my Heartseeker if I know they'll exhaust/blind. They think the fight is over and you hit them for half their HP. If you saved Aegis or if it's back up your stun will last until the exhaust is gone and it's gg for anyone in your face.
Heartseeker also provides some AOE damage. Yes, it's situational, but if your opponents are grouped around one of your teammates a good Heartseeker will scare off just about anyone.
Basically Heartseeker is much better burst. Personally, I take one rank of each skill, then max Heartseeker, then Spear, followed by Aegis. I know, Aegis lowers the CD, but it's still 10 seconds. If you're chasing enough to use it more than once as a 12-15 second CD, you're probably chasing too far. Consider also that more damage from your Spear Shot means you don't have to chase nearly as long.