Check the cancellation rules. R class on QR can sometimes be cancelled with a significant cancellation charge; I class can be completely inflexible. This may or may not make any difference, particularly if the fare is low in relation to the cancellation fees, but may be a concern.

Regarding rerouting, you might find it more difficult if on a 125 BA ticket than on a 157 QR ticket beforehand, but BA would own this until it was under airport control. Once travelling, I doubt that the issuing carrier would make any difference to your rerouting options during irrops.