I think it would be a vending machine.
Could be either.
Vending machine as in drop in $20 and get a code worth $20 of BTC. Or when price is more stable drop in $20 and get a private key with about $20 of btc.
As an ATM you are doing something more interactive such as both buying and selling. You could give the ATM machine a code via QR to send BTC too. Both models could work each with advantages.