As far as the Costco card I don't think it would be worth getting the card just for the cash back if you do not shop at Costco otherwise, but if you do, why not? It covers gas, dining, travel (when that happens!) and a lot of Costco shoppers get a big chunk of their groceries there.
But at least with the Costco card, although they encourage you to use it at Costco (by providing it as a reward voucher and forcing you to redeem in person), you don't have to. You can cash it out and do what you want with the cash.
If you're going to use Verizon anyway, the Verizon card offers a good rate of return if you don't mind just applying it towards your bill. I prefer more flexibility but ultimately, with cash back, it's going to come out to same thing. Your bill is lower so it frees up money for you to use on other things. No different than the eternal debate about whether statement credits are just as good as direct deposits, etc. for cash back. Psychologically I like the flexibility but practically speaking there is really no difference. You either pay less on a bill or have more money in your account.