When two boats share the same hull type — in this case both the Nitro 640 2008 and the Nitro Z-8 2009 are modified vee designs with aluminum construction — 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?

Size is the most obvious dividing line here. The Nitro 640 2008 measures 18,0 feet overall (2008), giving it roughly 16,0 additional feet of deck space compared to the Nitro Z-8 2009 at 2,0 feet (2009). Weight tells a clearer story for trailering families: the Nitro 640 2008 tips the scales at 1 092 lbs — 897 lbs more than the Nitro Z-8 2009 at 195 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.

The power gap is worth calling out. Rated to 250 hp, the Nitro Z-8 2009 has a 160-hp advantage over the Nitro 640 2008'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 Nitro Z-8 2009 carries 55 gallons versus 24 gallons in the Nitro 640 2008. On a lake day that's negligible, but for coastal cruising or long reservoir runs the extra range matters.

Both boats are rated for 4 passengers — a good fit for a family of four or five plus guests. Comfort at capacity is another matter; the longer hull typically means more seat options and better weight distribution.

At this size, power-to-weight ratio matters more than outright horsepower. The Nitro Z-8 2009 comes in at 1 lbs per hp versus 12 lbs per hp for the Nitro 640 2008. The lower the ratio the more explosive the acceleration — meaningful on a short RIB where bursts of speed, quick planing, and agility in surf or tight waterways define the experience.

The Nitro 640 2008 has a documented top speed of 33 mph. Speed data wasn't available for the other model.