Primal Jack · 9:18pm May 6th
Found this image courtesy of Reddit. It was too good not to share. 
Speaking a little more seriously though, it's interesting to look at this and compare/contrast the two characters' designs and the respective art styles of their shows.
Also by way of comparison, looking at Spear and at Jack, I had the interesting realization that their stories run as pretty neat contrasts to each other.
Spoilers to follow for anyone who cares:
Samurai Jack is a child when Aku attacks and destroys his home, enslaving his father and sending his mother into hiding; Spear is an adult who loses his wife and children to the red tyrannosaurs.
Jack is a man out of time, flung into an impossible future at the end of the first episode and looks for a way back to the past; while Spear starts out in his prehistoric homeland before arriving in a more advanced (if still anachronistic) continent without any time travel involved at all.
Jack is a loner for much of his series, even when he has allies like the Scotsman, whereas Spear and Fang's companionship is a cornerstone of Primal.
Jack ends the series having destroyed Aku for good but having to live with the loss of Ashi. Spear ends the series giving his life to stop the Viking chief-turned-Fire Demon, but still leaving a legacy through the child he sires with Mira. (To say nothing of Fang's hatchlings, for another kind of legacy.)
There may be more, but that's just all off the top of my head. Food for thought at any rate. 
Dang, now I kinda want to dig up some episodes of classic Samurai Jack again. 
Anyway, let me know your thoughts! 
There's also the nature of their enemies. Jack's problems either stem from Aku or are Aku directly intervening. Spear doesn't really have an archenemy so much as a savage world full of monsters and madmen to deal with.
Both men have the most epic eyebrows, though.
i.imgflip.com/2jdv9f.gif
static.wikia.nocookie.net/primaltv/images/1/18/NewFeederD.jpeg/revision/latest?cb=20201021155339
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That's another good point right there. Aku is just as much a central character to Jack's show as Jack himself. Whereas with Spear, the closest thing he has to a personal enemy is entirely from the other guy's POV. (I must say, sick as it is, there is something morbidly funny to me about how the Viking Chief shows up in the last ten or so minutes of the show to claim his revenge... and then gets instantly dragged back to hell a few seconds after achieving it. Mira's expression of "what the?" says it all.)
Also, yeah, true, both caveman and samurai have some awesome eyebrows going on there.
Wonder if he could also teach Jack to jump good.
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Hah, good question.