Bitcoin or Monero
The coin you pay in is one of the few choices on a darknet market that is purely yours, and it is the one with the longest tail. Bitcoin is easy to get and easy to trace. Monero is the opposite. Which one is right depends on what you are trying to protect, and this is the honest comparison.
The traceability gap
Bitcoin is a public ledger. A deposit is visible to anyone who looks at the address, and the chain of coins can be followed. Monero hides the amount, the sender and the receiver with ring signatures and stealth addresses. If the reason you are on a darknet market is to not be traced, paying in a coin that traces you is a contradiction worth noticing.
The practical side
Bitcoin is easier to obtain and the fee varies with congestion. Monero is slightly harder to source and the fee is steadier. Neither is a problem for a normal order. The decision is not about convenience, it is about how much the trail matters to you.
A reasonable middle
Some buyers mix Bitcoin before it reaches the platform, or route it through a service that breaks the chain. It adds a step, but it keeps the ease of Bitcoin with less of the trail. Whether that step is worth it is your call, and it is the only call that matters here.