Early on, Highlander just deals insane amounts of damage through Cartar Stroke; however, that damage starts falling off as levels keep getting higher.
Peltasta is the opposite: you start with a rough early leveling but once you get to higher ranks, where your damage starts getting good, then the ability to AoE taunt mobs is just incredible and totally worth it.
I am expecting Cartar Stroke to be nerfed in the future, as the game comes closer to release, so I think picking Highlander to level up faster abusing Cartar Stroke and get to see more out of the beta is fine. That said, I firmly believe that Peltasta is the sure winner at later stages of the game, even if Cartar Stroke remains as it currently is.