Discover and Capital One both had deposit-based credit cards for credit building, so that's what I started with.
Discover's customer support system was polished and felt like a small company. I'd always speak to a polite native English speaker. When I gave authentication information to the pre-call robot (like card number, name, exp date), the customer service rep could pull up that info instead of asking me to repeat it.
Also, my Discover card kept me up to date with it's upgrade status, eventually going from a $150 credit line to a $1500 automatically and predictably. And again going to a $2500 credit line without my intervention; just from making steady payments.
Capital One's customer support system was awful. It's outsourced and I can't always understand the rep who sometimes gets mad at me for it. Any authentication information I give to the pre-call robot, the rep will ask me for again. Even the dang credit card number that is hard to give to a human. "No, that's Two as in Todd, Four as in Forage..."
And the Capital One credit card I got 3 years ago is still at $150 credit line. Apparently I have to pester them to get the credit line upgrade that the card promises, but why bother with a crappy company when I have a $2500 Discover card and good enough credit to have gotten better cards since then?
But I guess it's no surprise that the worse company to work with is swallowing the pleasant company. Just imagine all that money Discover was wasting by being a pleasant experience, like having credit lines that upgrade automatically and a customer support rep that speaks the same language.