The answer would be pretty dependant on how much Stamina and Weapon Damage you normally have. Let's take some genaric values to compare:

Say I have 30k Stamina and 3k Weapon Damage.

30000 x 0.04 = 1200
3000 x 0.06 = 180
180 x 10.5 = 1890
1890 - 1200 = 690 difference

In this example, the power of the bonus weapon damage mathematically converted to a magicka buff is equivalent to giving 690 more magicka. However, this only ia applicable to attacks from stealth and does not actually increase you mac Stamina pool at all. If you intend to be relying on stealth and ganking, probably stick with the extra sneak damage.

I just checked and the bonus from,stealth is actually 10% so thanks again. I will stick with stealth bonus for now.