We've sailed on both lines and liked them both for different reasons. An additional difference I would point out is that RCCL will carry a much higher passenger load than HAL. While there are more fun activities on the RCCL ships, there will certainly be a line of passengers waiting to do them (assuming you're sailing in high season). The same will be true for things like getting off the ship in ports of call, booking excursions, spa appointments, getting a seat at the $5 blackjack tables, etc.
On the HAL ships this simply does not happen.