Thureos Spears vs Peltasts - What's the difference?

Obviously, there is a difference between the two units, especially when it comes to different factions, but from what I gather, both Thureos Spears fill in the same niche as Peltasts. That is to say, they wait on the flanks of your force until the lines meet and then kite around to start throwing javelins into the enemy's rear before charging in. Now I do know that both can, under certain circumstances, be used as frontliners if needed, and that the Thureos spears have spears which would make them better for supporting the cavalry, while the Peltasts would be better as skirmishers/flankers, but ultimately I feel conflicted on which one to bring into battle for my Hellenic armies. Is it mostly down to preference/which enemies you're facing? Or is there a clear winner and loser that I'm just not seeing?

This is for SP btw, though your MP reasoning would also be welcome.