In 8th, the trade off was go last, but hit hard every turn, or hit hard on the charge, and get a better save, with a slight more chance of going first, not to mention to get great weapons, you had to kitbash them, so more often you went with lances anyway.
In AoS it's a different trade off. Lances have 2" range, meaning you can attack with two ranks, if you're still ranking up your models, but that is negated by the 2 attacks for using a cav hammer anyway, and cav has a better to wound roll.
The lances get +1 to wound and +1 damage on the charge. And once per battle can re-roll hits
Reiksguard get +1 to hit over normal knights
I think I would go with cavalry hammers, and the double attacks. Difference in to and to wound isn't anywhere near as good as double attacks. double damage on the charge is pretty much the same as double attacks, but double attacks is every turn, not just charge turn