Both the Key West 1720 CC 2011 and the Key West 218 SK 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?
The power gap is worth calling out. Rated to 150 hp, the Key West 218 SK 2009 has a 30-hp advantage over the Key West 1720 CC 2011's 120-hp ceiling - enough to notice on acceleration and at cruising speed, particularly with a full passenger load. Both carry nearly identical fuel loads - 31 gal and 30 gal - so range won't be a tiebreaker here.
The Key West 1720 CC 2011 is rated for up to 5 people. Passenger data for the Key West 218 SK 2009 wasn't available.
Bottom line: The Key West 218 SK 2009 at 21,0 ft is the right call if deck space, comfort, and entertaining are top of your list. The Key West 1720 CC 2011 at 17,2 ft wins on trailering ease, likely lower purchase price, and simpler docking - a solid choice for a buyer who wants more boat for less money.