Also, more generally than just choice of curve, the CryptoNote coins (of which Monero is one) are rare amongst cryptocurrencies in sharing very little of the cryptographic design of Bitcoin.  So in the event that a flaw is found in Bitcoin's cryptographic design, there's a reasonable chance it might not affect Monero (or vice versa).

The one obvious advantage that Bitcoin does currently have over CryptoNote is that its design (which has remained fundamentally unchanged since it's initial launch) has been very widely studied over the years since then.  Generally this kind of widespread review is the only way to gain assurance that there are no flaws in a cryptographic protocol, and Monero has inevitably had fewer cryptographic eyes on it.

Of course, in the very near future Bitcoin is likely to deploy segwit.  Although segwit doesn't change the underlying cryptography, a significant change to cryptographic protocol always carries a risk of unintended consequences - so it becomes harder to argue that Bitcoin, in its post-segwit future form - will have had the same number of eyes on it.