Is it better to disenchant something with +30% something, or doesnt it matter if i use +15% or 20% ?

I've always disenchanted lesser items if I have a choice. I see no reason why there would be any benefit in purposefully doing it on something I might consider using.
You only learning the magical enchantment option, not it's power.

it doesn't matter if you disenchant something that gives a +10 or a +50. when you go to enchant an item, the corresponding rating will be based on your enchant skill, perks selected, and soul gem type. so you could disenchant a +10, but with a maxed out enchant skill and tree, it would end up being a +50.

This.

You learn the enchantment no matter what the power of the enchanted item. Also once you disenchant the item it is destroyed so there is no point destroying a high powered enchanted item if you have a lower enchanted item....especially if your enchanting level isn't high enough to replace the enchanted item you destroyed.

Aye, always use the lower powered items if you can help it.

I wore a +15% archery ring until I was a good enough enchanter to disenchant it and make it a +30%.

There is one exception: disenchanting the Shield of Solitude will reportedly create a separate and more powerful base version of a Resist Magic enchant than disenchanting most (all!) other Resist Magic gear.

Thnx guys!