spear is an instant cast skill with a gaurenteed hit, and is better to finish off enemies.
I have played a LOT of pantheon, both Q and E first (and even tried W first, not worth it by far).
E first, most of the time you won't hit all strikes, often not enough to be truly effective. The spear on the other hand allows you to throw spear and then go into the HSS.
As HSS is better strengthened thorugh bonus AD, and since spear is so much more available (you can always throw a spear, most of the time a good enemy will not let you get off a full HSS).
Should you crit with a spear, the damage is equal for lvl 1 spear + lvl 5 HSS, or lvl 1 HSS and 5 spear. But you have to land all the hits of the HSS to do the same damage. Whats more, if you look at non crit, you are sacrificing about 80 damage in total by maxing spear first. But if you were to miss ONE strike, you would be dealing more damage with spear. Whats more, spear you can throw and run away, HSS holds you in place allowing less kiting.
From experience playing against top teams in tournaments, I would strongly suggest Q first, simply because it is more effective in the level 1 fight, and if you are playing pantheon you will probably dominate at least slightly, and the HSS will get interrupted/avoided very efficiently