If you don't own a Schwab or DKB card, be prepared to get charged (from your Bank) per transaction. Also, what do you mean with debit card? If it's not a proper Visa/Mastercard/Amex, it might be VERY possible that it won't work at all. Have you checked that with your bank, that it will work? I assume (reading your other post in this subforum) that you've no experience travelling abroad, that's why I'm asking.

Regarding getting RMB (Yuan) in China:

-ATMs are good (and the rate is basically the same everywhere +/-) if you're having a card with low or no fees.
-Cash or Traveller Checks might get you slightly better results, but can be big pain in the ... as Banks aren't open on weekends, hotels will change cash at crap rates (like pretty much everywhere in the world) for the convenience, and traveller checks are usually only accepted at Bank of China, sometimes even just at their local MAIN office.. - some will say that cash works fine for them being able to change "everywhere", but keep in mind some of that might be actually illegal (change on the road with a stranger.. not to say there is a risk of counterfeit bills..)

In short: I prefer ATMs while using no-fee ATM card, which enables me to just get 500-1000 RMB in a single transaction, so I'm not a "big fat target".. also, the biggest denomination is 100 RMB, which means if you get 500US$ in RMB, you'll get 30 bills at least. Makes for one hell of a thick purse

But back to your situation: IF your card works in China (please check that beforehand!) then check the charge on each withdrawal. If it's not too high (say, 5$/withdrawal) but not too little either, then always "max" the ATM (usually the maximum is 2000 RMB, but I've seen higher/lower ones as well)

And it won't make a difference at what ATM you're using the card, no matter if Beijing or Guangzhou (there are plenty ATMs in PEK airport, and 3 hours will be plenty of time especially when arriving/departing at the same terminal)

And yes, as you noticed, it's a horrible way to change for RMB while outside of China, just avoid that.