Either ride first thing or FP will work fine. The nice thing about the FP approach is you have a spot on the ride later if it happens to go down. If you are in the regular standby line and it goes down then you will lose your spot usually.
It also depends on whether you are early arrivers. It is not worth going on RSR first thing unless you arrive at the DCA gate at least 30 minutes early. The line builds so quickly in the morning that if you are not towards the front you will have a pretty long wait in standby.
If you think you might want to ride RSR twice on that day then your best bet is to arrive early, ride RSR, then get a FP. If you time the FP window properly your second ride can be at night which feels like a different experience.
If you go the FP route only (and thus just one ride) I would encourage you to get the FP for earlier in the day in case the ride does go down. In that case you have the rest of the day available for it to come back and use your FP. If the ride is down during your FP window you can come back any time that day to ride it with the FP.