[Guardian's Pendant of Subjugation] is one of the best necks until Sunwell, narrowly beating
[Loop of Cursed Bones] when socketed correctly.
[Guardian's Band of Subjugation] on the other hand is terrible, as in worse than Kara equivalents.
[Guardian's Band of Dominance] is better, but may be problematic to get if you are not an arena junkie (1650 rating requirement).
[Belt of Blasting] is superior to the new
[Guardian's Silk Belt], although the materials can be pricy depending on your AH.
I imagine your friend is advising against gearing for haste because 1) original T4 and T5-level itemisation didn't feature it, so it has become associated with high-end raiding only, 2) haste is not a very powerful stat for arcane specs, and T5 clearly is designed with arcane mages in mind and 3) it depletes your mana faster than if you spent equivalent item points on extra +dmg or +crit. However, for frost/fire specs, spell haste is always better in terms of pure DPS than crit and is competitive with spell damage for any realistic gear level, assuming you can support the mana drain (i.e. do you have a shadow priest).
There is no simple answer as to when haste is better than plus damage, as this depends on your base +dmg/haste level, your group composition, raid buffs, and spec. In terms of gear upgrades, however, the question is usually fairly clearcut. Rawr will be able to help you with the specifics here.
I'd advise you to buy
[Carved Witch Doctor's Stick] as it is cheap, and get the aforementioned
[Belt of Blasting] crafted to free up some spellhit. The new badge gloves or robes would be good choices to upgrade too, although the badge cost is high. If you have money to burn assembling a Crusades deck to replace your
[The Lightning Capacitor] is not a bad idea either.
Edit: Also, unless you primarily use Mage Armor, Vitality to boots (4mp5/hp5) is better regen than 5 spirit. Even if you do use Mage Armor I suspect Vitality will be better given your low spirit value, but without running the calculations I'm not 100% sure.