Awazon takes Bitcoin and Monero, and the right choice comes down to what you are optimising for.
Monero, the privacy default
Monero hides the sender, the receiver and the amount by design, so the payment is not legible on a public ledger. If privacy is any part of why you are on Tor, this is the coin to default to, and most buyers should.
Bitcoin, when required
Bitcoin's ledger is fully public, so a payment in it leaves a permanent readable trail. Use it when a vendor accepts only Bitcoin. If you do, do not send straight from an identity-checked exchange, because that ties your name to the deposit. Route through a fresh wallet or swap into Monero first.
The practical answer
Default to Monero. Reach for Bitcoin only when a vendor forces it, and even then consider swapping into Monero before you deposit.