Currency based certificates are not notoriously hard to use since they function just like "cash" (no capacity controls etc). As to the redemption process - they vary by airline, some deposit them into an "account" tied to your frequent flyer number, some will hold the reservation so that you can mail in the certificate, or you can redeem them by ticketing at the airport (United will do that with no airport ticketing fee).

For me, the $500 cert vs the 22,500 miles is no contest, I'd take the $500 cert every time. From my home airport, $500 would pay for 1.5 roundtrips to the west coast, so taking those flights would earn me approximately 12,000 miles. While the 22,500 miles wouldn't even get me 1 rountrip since that requires 25,000 miles (and heavily capacity controlled) and flying on an award ticket earns you no miles. So the net comparison for me becomes $500 + 12,000 miles compared to 22,500 miles.