Both the Go-Devil 18 x 56 2013 and the Go-Devil 20 x 54 SD 2011 are flat-bottom 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 - Go-Devil 18 x 56 2013 at 18,0 ft versus Go-Devil 20 x 54 SD 2011 at 20,0 ft. At 580 lbs and 660 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.
Both boats share a closely matched power ceiling - 50 hp for the Go-Devil 18 x 56 2013 and 50 hp for the Go-Devil 20 x 54 SD 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 Go-Devil 20 x 54 SD 2011 is rated for 6 passengers, while the Go-Devil 18 x 56 2013 caps at 5. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Go-Devil 20 x 54 SD 2011 could be the deciding factor.
Bottom line: Choose the Go-Devil 20 x 54 SD 2011 if your priority is putting more people on the water - it handles 6 passengers and at 20,0 ft it has the deck room to back that rating up comfortably. The Go-Devil 18 x 56 2013 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 5 that costs less to run day-to-day.