You are correct that the most comparable itinerary from Port Canaveral is the Sovereign. The Sovereign underwent an extensive "revitalization" drydock in December 2004 that was filmed and shown as a multi-part mini-series on both Discovery Channel and Travel Channel. My impression is that the ship is in very good condition, and that was based on my impressions comparing the Sovereign in 1/07 with the Radiance in 11/06--I did not feel a drop off in caliber of ship, and there was just a 5 week span in between.
That said, there are some significant differences. 1) Stateroom size. Tiny on Sovereign, at 122 feet for std. oceanview, vs. 226 for a cat. 9. We had a Jr. Suite, at 245 ft. plus a balcony, the room is really like a cat. 6. 2) No alternate restaurant. Newer RCI ships, like Mariner, have a Portofino or a Chops, filling the same role as Palo, but not on Sovereign. 3) Price--as OP has noticed. We have pretty consistently been more impressed with RCI's Windjammer Buffet restaurants vs. Beach Blanket/Topsiders, though we give the edge to
DCL for the dining in the rotational resturants. Though a smaller ship at approx 73,000 tons, the sovereign, when full, holds about 2600 passengers! And just two hot tubs. Other threads compare other differences, such as entertainment and children's programs between the two lines.
Sovereign can be a very nice experience. No, it is not a
Disney cruise, but it is a very nice cruise--but the details will be different. If it fits your budget--and plan on 2 staterooms on the sovereign, then consider it.
We have also done a 4 night Enchantment cruise from Ft. Lauderdale. Another revitalized ship, this is the one that was cut in half, and had an extra middle section added in 2005. We cruised on her 2/2006. Rooms are larger than the sovereign, though still not the same size as DCL. This one does have a Chops. No basketball court (DS's favorite--Sovereign does have a court, FYI). Typically, no private island on the Enchantment itineraries. RE: private islands--coco cay on RCI is perfectly nice, but
Castaway Cay wins, for the disney touches and being able to dock and walk on and off w/o tenders.
There may be fewer families on Sovereign or Enchantment except for peak holiday weeks when school is out, but neither was a booze cruise with anyone out of control or unpleasant in any way. Experienced large group booking on both ships, FWIW, so for kids a somewhat different atmosphere--but plenty of family activities on sovereign, perhaps even a bit more than on Enchantment. Enchantment also offers the grownups a covered pool solarium in case of cooler weather during your cruise.
Have you considered checking the prices for something w/o a balcony on DCL to see the pricing? We've always loved our cat. 8's, and even having had balconies before find no problem in going to a cat. 9 for our upcoming Wonder cruise. In any event--in doing a price comparison, plan on 2 rooms for sovereign, and very possibly the same set up on enchantment.