Per effects Slow is much more powerful than Haste.
But:
Slow needs to bypass SR and a save.
Haste targets are usually willing.
Haste can be easily cast on everyone because you grouphug for it but Slow is rather hard to land on everybody because enemies often don't grouphug for you.So:
You are a specialist wizard with Spell Penetration and Spell Focus and face a lot of enemies? Slow ruins their day.
You are in a mundane damage heavy party? Haste wins the day.
You are the DM? Nothing is more fun than debuffing the Party and see them suffer. Slow wins the day.
Haste also makes you immune to slow, in the sense you are sacrificing an easier to land lesser effect to prevent or repair the damage done by the harder to land slow.
Haste is also a power multiplier. It works on your party, your corhorts, your animal companions, your undead minions (and you should have undead minions :P), your planar bound minions, etc, etc. :3
By 20th level, a wizard can drop a quickened haste and hit 20 targets with it. That's easily enough for them, their party, their party's animal companions, a couple of bruisers, your simulacrums, your simulacrum's simulacrums, and a couple of planar bound critters. ;P
Of course, I actually think that Haste & Slow are pretty even. Since heightened persistent slow ruins days at high levels. Hell, it would make AM BARBARIAN a bit sad too. :P
But boy, could you imagine if it was like Improved Haste from Baldur's Gate I & II (found in the oils of speed potions everywhere)? That spell was epic. It DOUBLED your attacks per round, in addition to making you faster. God that spell was so sick. Your Fighter has 3 attacks per round? Time to turn him into a blender! Of course, that version of Haste only affected 1 person. :P
Haste, for all the previously-mentioned reasons, but also because the speed boost really helps with tactical positioning. Maybe a lot of folks don't use map and minis--people tend to overlook the utility of movement.
The +30 ft speed is also +12 to Acrobatics checks for jumping. Just tossing that out there. Oh, and it also means using Stealth at 30 ft. speed with no penalty, or at 60 ft. speed with a -5. Your ranger and rogue companions will often enjoy this while Stealthing in combat.
Still the effects of Slow are better than the ones of Haste.
That was my whole point. It is en par with haste after considering the save and SR.
Also Haste is not that good for Animal Companions because you don't get extra attacks with them. And most Barbarians will save against it too because they have superstitious.
But Haste really depends on how many mundane chars you have. Your party has a fighter, a barbarian and a paladin? Haste rocks! Your party has a witch a cleric and a sorcerer? Haste is... ok...
Slow is a 3rd level spell. Expecting people to fail it past a certain point will make the caster sad. Haste is good for all 20 levels. Most monsters at higher levels get good will save, and they have SR. That basically doubles the chances failure, and chances are not that good to begin with.
If you heighten, and persist Slow it has a better chance, but if you have to do that much to make it last for all 20 levels then I think haste has won the day, especially since the metamagic'd slow can still be shut down by a scroll of haste.
Slow is a 3rd level spell. Expecting people to fail it past a certain point will make the caster sad. Haste is good for all 20 levels. Most monsters at higher levels get good will save, and they have SR. That basically doubles the chances failure, and chances are not that good to begin with.
If you heighten, and persist Slow it has a better chance, but if you have to do that much to make it last for all 20 levels then I think haste has won the day, especially since the metamagic'd slow can still be shut down by a scroll of haste.
Welcome to our crappy Vanciant spellcasting.
You would need to have one and the same aggressive spell about 3 times in your class spellist to make it useful.
On the other hand a buff spell only gets more and more overpowered as you advance levels.
Good thing Paizo didn't change a thing here :)