Both the Sundance NX19 2011 and the Sundance SV171 2009 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 2011 at 18,8 ft versus Sundance SV171 2009 at 17,1 ft. At 1 475 lbs and 1 430 lbs respectively, both sit in a similar weight class - either should pair comfortably with most mid-size SUVs and half-ton trucks, though always confirm your specific tow rating with the motor added.
The power gap is worth calling out. Rated to 115 hp, the Sundance NX19 2011 has a 25-hp advantage over the Sundance SV171 2009's 90-hp ceiling - enough to notice on acceleration and at cruising speed, particularly with a full passenger load. Fuel capacity breaks the other way: the Sundance SV171 2009 carries 34 gallons versus 29 gallons in the Sundance NX19 2011. 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 NX19 2011 is rated for 6 passengers, while the Sundance SV171 2009 caps at 5. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Sundance NX19 2011 could be the deciding factor.
Bottom line: Choose the Sundance NX19 2011 if your priority is putting more people on the water - it handles 6 passengers and at 18,8 ft it has the deck room to back that rating up comfortably. The Sundance SV171 2009 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 5 that costs less to run day-to-day.