You can catch the earlier or later bus, but only if there are seats available and the coach is not full.
If you are arriving to BCN at 9:45 I would recommend you to book your seats for the 12:15 bus, as 30 minutes is very short time to pick up your lugagge, cross the passport control and go out of the airport.
That way you will have your seats warrantied for the 12:15 bus. In case you arrive earlier to the airport and you have enough time to be at 10:15 at the bus stop you could also catch this one if there are seats available.
Please, let me know if you have any other question,
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