The magnitude of the effect on the item you disenchant does not impact the magnitude of the effect on items you enchant yourself. It depends entirely on your skill, the soul gem used and your enchanting perks.

So, given the choice between disenchanting an item of sneak 10% or sneak 45%, you would disenchant the item of sneak 10% to learn the fortify sneak enchantment and save the item of sneak 45% to either sell or keep.

The magnitude of the effect on the item you disenchant does not impact the magnitude of the effect on items you enchant yourself. It depends entirely on your skill, the soul gem used and your enchanting perks.

So, given the choice between disenchanting an item of sneak 10% or sneak 45%, you could disenchant the item of sneak 10% to learn the fortify sneak enchantment and save the item of sneak 45% to either sell or keep. If you are trying to level your Enchanting skill you may wish to disenchant the equipment with the highest effect as this will give more exp.