Awazon takes two coins, and the choice is simple. Use Monero when privacy is any part of why you are here, because it hides the sender, receiver and amount, so the payment is not legible on a public ledger. Use Bitcoin when a vendor accepts only Bitcoin.
Most buyers should default to Monero. Its ledger does not leave the permanent public trail that Bitcoin's does, which is the whole reason many people are on Tor in the first place. Bitcoin remains fully supported for the vendors who have not switched.
Whichever you pick, the next sub-steps cover getting it to the market cleanly.