Greater soul gems will grant more xp than petty.
That might be true for recharging items, but for straight-out enchanting at the alter, all enchantments, souls, and soul gems all provide the same amount of XP.
I personally buy petty/lesser (and sometimes common) soul gems to increase enchanting in bulk. I go raiding and use my soul trap weapons to harvest souls and then enchant several dozen rings/necklaces/armour pieces at once.
Getting the black star and then recharging your weapon with it after every kill is also a fast way to increase enchanting, though I prefer the above...
-HK
EDIT: http://www.uesp.net/wiki/Skyrim:Enchanting
Gaining Skill XP
- Crafting enchanted items gives the biggest overall skill increases. The increase is a flat number per item.
- Disenchanting items gives widely varied skill increases, dependent on the effect learned and the magnitude on the item, with apparel effects generally giving larger increases than weapon effects. The base value of the destroyed item is irrelevant. However, only one item per http://www.uesp.net/wiki/Skyrim:Enchanting_Effects may be ever disenchanted.
- Recharging enchanted weapons using http://www.uesp.net/wiki/Skyrim:Soul_Gem gives small to moderate skill increases, based on the size of the soul used. It does not matter how many charges there are left on the weapon. Using http://www.uesp.net/wiki/Skyrim:The_Black_Star and a Weapon with Soul Trap effect, you can train by recharging the weapon after every kill without having to worry about soul gems.
One way to level up Enchanting quickly is by disenchanting items - the more powerful the better - but will only get you so far. After that, you will have to advance by crafting items: acquire a number of filled soul gems of any size and the same number of unenchanted weapons or armor. Enchant all of them; preferably with the most expensive enchantment you know for maximum profit. Using http://www.uesp.net/wiki/Skyrim:The_Mage_Stone and a Sleeping bonus (such as Well Rested) will increase the experience you gain per enchantment.