When two boats share the same hull type — in this case both the Weldcraft Marine 18 SJ 2009 and the Weldcraft Marine 21 2011 are deep 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?
On paper these two are close siblings in the size department — Weldcraft Marine 18 SJ 2009 at 18,1 ft versus Weldcraft Marine 21 2011 at 21,0 ft. Weight tells a clearer story for trailering families: the Weldcraft Marine 18 SJ 2009 tips the scales at 205 lbs — 177 lbs more than the Weldcraft Marine 21 2011 at 28 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.
For family outings this is probably the sharpest distinction between the two. The Weldcraft Marine 21 2011 is rated for 6 passengers, while the Weldcraft Marine 18 SJ 2009 caps at 1. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Weldcraft Marine 21 2011 could be the deciding factor.
Bottom line: Choose the Weldcraft Marine 21 2011 if your priority is putting more people on the water — it handles 6 passengers and at 21,0 ft it has the deck room to back that rating up comfortably. The Weldcraft Marine 18 SJ 2009 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 1 that costs less to run day-to-day.