Pete’s normal crit at high ranks is already above the level needed to guarantee a crit for his normal damage attacks; using 2 mods with normal crit upgrades maxed pushes him to the point where, at Y19, he’s got about a 90% chance at a super crit, which triples the damage from his normal damage attacks.
A difference of opinion. The purple skill is used, at most, twice per battle, but deals more damage and is used earlier in the battle (for at least the first time it’s used, anyway); it’s also possible that one skill or the other scales better (i.e., is increased more drastically) with the addition of relevant SP mods. Blue skill mod upgrades are also in shorter supply than purple ones, which may affect some peoples’ decision.
As Bluebandit93 pointed out, this thread would have been a better place to ask, and if you have any further questions about these mods it’s probably better to ask them there as well.