Both the Avalon 18 ft. GS Quad Lounge 2013 and the Avalon Tropic - 20 Foot 2008 are pontoon 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 - Avalon 18 ft. GS Quad Lounge 2013 at 18,0 ft versus Avalon Tropic - 20 Foot 2008 at 20,0 ft. Weight tells a clearer story for trailering families: the Avalon Tropic - 20 Foot 2008 tips the scales at 1 870 lbs - 270 lbs less than the Avalon 18 ft. GS Quad Lounge 2013 at 1 600 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.
The power gap is worth calling out. Rated to 90 hp, the Avalon Tropic - 20 Foot 2008 has a 30-hp advantage over the Avalon 18 ft. GS Quad Lounge 2013's 60-hp ceiling - enough to notice on acceleration and at cruising speed, particularly with a full passenger load.
For family outings this is probably the sharpest distinction between the two. The Avalon Tropic - 20 Foot 2008 is rated for 9 passengers, while the Avalon 18 ft. GS Quad Lounge 2013 caps at 8. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Avalon Tropic - 20 Foot 2008 could be the deciding factor.
Bottom line: Choose the Avalon Tropic - 20 Foot 2008 if your priority is putting more people on the water - it handles 9 passengers and at 20,0 ft it has the deck room to back that rating up comfortably. The Avalon 18 ft. GS Quad Lounge 2013 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 8 that costs less to run day-to-day.