Both the Mariah R19.9 2010 and the Mariah SX21 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 - Mariah R19.9 2010 at 19,2 ft versus Mariah SX21 2009 at 20,0 ft. At 3 650 lbs and 3 600 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 270 hp, the Mariah SX21 2009 has a 45-hp advantage over the Mariah R19.9 2010's 225-hp ceiling - enough to notice on acceleration and at cruising speed, particularly with a full passenger load. Fuel capacity breaks the other way: the Mariah SX21 2009 carries 44 gallons versus 38 gallons in the Mariah R19.9 2010. On a lake day that's negligible, but for coastal cruising or long reservoir runs the extra range matters.
Both boats are rated for 10 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.
Bottom line: Performance buyers should lean toward the Mariah SX21 2009 and its 270-hp ceiling. If fuel economy and quieter running matter more than top-end speed, the Mariah R19.9 2010 with its 225-hp rating is the more economical daily driver.