Both the Stanley Boats Islander 23 ft. 2012 and the Stanley Boats Islander I/O 21 2011 are deep-V designs, both built in aluminium, 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 - Stanley Boats Islander 23 ft. 2012 at 23,3 ft versus Stanley Boats Islander I/O 21 2011 at 21,0 ft. Weight tells a clearer story for trailering families: the Stanley Boats Islander I/O 21 2011 tips the scales at 2 765 lbs - 565 lbs less than the Stanley Boats Islander 23 ft. 2012 at 2 200 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.
Both boats share a closely matched power ceiling - 200 hp for the Stanley Boats Islander 23 ft. 2012 and 220 hp for the Stanley Boats Islander I/O 21 2011. Real-world performance will come down more to which motor is actually bolted on, its load at the time, and whether it's a 4-stroke or 2-stroke setup.
For family outings this is probably the sharpest distinction between the two. The Stanley Boats Islander 23 ft. 2012 is rated for 10 passengers, while the Stanley Boats Islander I/O 21 2011 caps at 6. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Stanley Boats Islander 23 ft. 2012 could be the deciding factor.
Bottom line: Choose the Stanley Boats Islander 23 ft. 2012 if your priority is putting more people on the water - it handles 10 passengers and at 23,3 ft it has the deck room to back that rating up comfortably. The Stanley Boats Islander I/O 21 2011 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 6 that costs less to run day-to-day.