they are sister ships, but the Majesty had an overhaul just a couple of years ago so it may be in better condition. Prices are almost the same, Monarch is slightly lower (about $50 for inside)
One consideration is the time in ports if that is important to you. Monarch sails out of Port Canaveral and may have a little less time in port on the first and last days because of the additional distance. Majesty sails out of Miami so the first and last days are longer port stops.
I have not been on the Monarch, but sailed her about 6 years ago and am going on the Majesty again in october.