Contrast with
Slow, a CORE 3rd level transmutation.
Action:
Unluck - Standard
Slow - Standard
Advantage? Neither
Save:
Unluck - Will negates, Divination school
Slow - Will negates, Transmutation school
Since a wizard is more likely to have Spell Focus: Transmutation than Spell Focus Divination (since there are only like, 6 Divinations with saves, vs hundreds of Transmuations with saves), Slow will often be slightly higher.
Advantage? Slow
Targets:
Unluck - Single
Slow - 1/CL within 30' of each other
Advantage? Slow
Range:
Unluck - Close
Slow - Close
Advantage? Neither
Duration:
Unluck - Rounds/level
Slow - Rounds/level
Advantage? Neither
Effect:
Unluck - subject rolls attacks and saves twice. If the subject hits on a 2+ already, thats only a slight increase in effect. If you were targetting a save with a save vs die spell, then the subject of Unluck would have already failed the first save if you had cast the other spell first, and could STILL save twice even with Unluck up. Doesn't really affect casters using non-attack roll spells.
Slow - subject is denied full attacks. Effective at low levels vs multiattackers like animals or TWFing rogues, effective at high levels vs anything with iteratives. Again, doesn't really affect casters.
Advantage? Slow
So really, Unluck is completely pointless as a debuff, and completely pointless as a set-up for another kill spell. If you manage to land the Unluck, you probably would have landed the kill spell. Even if you land the Unluck, you are STILL not garunteed to land the kill spell. Really, you'd be best off casting Slow twice in succession. If the first Slow works, the target is affected and the 2nd isn't needed. If the first fails, the second has a chance to work. Contrast with Unluck; if the Unluck works, you still have to cast the Slow because Unluck is a rather weak debuff. If the Unluck fails, you are in exactly the same position you'd be in if the first slow had failed.
Trust me, Unluck is a TERRIBLE spell. It should either be a swift action spell (and thus take almost no action, similar to Assay Spell Resistance) or not allow a save (similar to Curse of Impending Blades). While it has both of those features, it is catagorically WORSE than Unluck in nearly every catagory. Period.