This may shock some people (not many, but some), but these sites tend to have the same tickets available, from the same people.
The key is to compare sites and check the pricing. Take advantage of discounts codes, referral fees, etc.
If you find a ticket on stubhub, and the price on tickpick (or another site) is cheaper, then you should buy it on tickpick and save yourself money.
If a seller on stubhub is legitimate, he is probably also legitimate on tickpick. I'm pretty sure both sites offer equivalent guarantees on the tickets.