I've done both. The rebate company says they can't do anything for another month (typical brush off) and Corsair won't get back to me.
BTW Corsair is the only company that's ever sent me a prepaid credit card for a mail-in rebate. All I've ever received from anybody else (AND from Corsair in the past) were cheques.
Correction. I did actually receive a prepaid card for a mail-in rebate once, from Asus. It was an American Express prepaid card, issued in my local currency, with none of those BS fees. It actually worked, first time, for the full amount of the mail-in rebate.
So I know it CAN work, it's just that yours don't. Does that make the situation any better?