1. There's no such thing as global currency.
2. Some countries' currencies may get replaced by Bitcoin, but seeing all of them replaced by Bitcoin is something I don't expect to happen during my lifetime.
3. Bitcoin is already getting used as a reserve by certain companies, or at least that's how I see their moves (one example being Tesla's). i think that's only a mater of time to see this getting extended to central banks.

So it can be both, but globally, Bitcoin-as-reserve has better chances than Bitcoin-as-currency.