Both the Sundance NX19 2012 and the Sundance NX21 2010 are modified-V designs, both built in fibreglass, so the buying decision usually comes down to a handful of practical questions: how many people are you putting on the water, how far do you trailer, and what does your tow vehicle weigh?
On paper these two are close siblings in the size department - Sundance NX19 2012 at 18,8 ft versus Sundance NX21 2010 at 21,1 ft. Weight tells a clearer story for trailering families: the Sundance NX21 2010 tips the scales at 1 850 lbs - 375 lbs less than the Sundance NX19 2012 at 1 475 lbs. That difference is meaningful if you're working within a half-ton or three-quarter-ton truck's tow rating, especially once you factor in a motor, gear, and fuel.
Both boats share a closely matched power ceiling - 115 hp for the Sundance NX19 2012 and 115 hp for the Sundance NX21 2010. Real-world performance will come down more to which motor is actually bolted on, its load at the time, and whether it's a 4-stroke or 2-stroke setup. Fuel capacity breaks the other way: the Sundance NX21 2010 carries 45 gallons versus 29 gallons in the Sundance NX19 2012. On a lake day that's negligible, but for coastal cruising or long reservoir runs the extra range matters.
For family outings this is probably the sharpest distinction between the two. The Sundance NX21 2010 is rated for 8 passengers, while the Sundance NX19 2012 caps at 6. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Sundance NX21 2010 could be the deciding factor.
Bottom line: Choose the Sundance NX21 2010 if your priority is putting more people on the water - it handles 8 passengers and at 21,1 ft it has the deck room to back that rating up comfortably. The Sundance NX19 2012 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 6 that costs less to run day-to-day.