Vs IG: Mark of Khorne upgrade can be great, and versatile against IG, depends if the IG still has sentinels up. If he has gone sentinel heavy and kept them alive through T1, then Mark of Tzeentch is a good upgrade. Use them in conjunction with heretic worship.
Vs: SM No brainer, MARK OF TZEENTCH, MARK OF TZEENTCH, and... more MARK OF TZEENTCH - and an aspiring champion for extra dps.
Vs Chaos: Same as the above
Vs nids: keep them as a 'naked' squad, two upgrades are not that useful, buy an aspiring champion to go toe to toe with hormagaunts.
Vs Eldar: Mark of tzeentch is a great upgrade against eldar, it shreds guardians, banshees, and basically every single eldar infantry squad. Dont go Mark of Khorne as Banshees with either one of their aspects will win head to head.
Vs GK: Same as SM and Chaos