When two boats share the same hull type — in this case both the Duffy 18 Snug Harbor 2013 and the Duffy 21 Sun Cruiser 2013 are modified vee designs with fiberglass 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?
On paper these two are close siblings in the size department — Duffy 18 Snug Harbor 2013 at 18,4 ft versus Duffy 21 Sun Cruiser 2013 at 21,0 ft. At 27 lbs and 4 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.
For family outings this is probably the sharpest distinction between the two. The Duffy 21 Sun Cruiser 2013 is rated for 12 passengers, while the Duffy 18 Snug Harbor 2013 caps at 11. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Duffy 21 Sun Cruiser 2013 could be the deciding factor.
Bottom line: Choose the Duffy 21 Sun Cruiser 2013 if your priority is putting more people on the water — it handles 12 passengers and at 21,0 ft it has the deck room to back that rating up comfortably. The Duffy 18 Snug Harbor 2013 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 11 that costs less to run day-to-day.