The Marathon Grumman Journey 2085FP 2013 is a pontoon design and the Marathon Seneca 12 2012 is a modified-V design, so you're likely deciding between two noticeably different on-water experiences. Hull type shapes everything from ride quality and fuel burn to dock handling and resale trajectory.
Size is the most obvious dividing line here. The Marathon Grumman Journey 2085FP 2013 measures 20,0 feet overall (2013), giving it roughly 8,3 additional feet of deck space compared to the Marathon Seneca 12 2012 at 11,7 feet (2012). Weight tells a clearer story for trailering families: the Marathon Grumman Journey 2085FP 2013 tips the scales at 1 890 lbs - 1 769 lbs more than the Marathon Seneca 12 2012 at 121 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 100 hp, the Marathon Grumman Journey 2085FP 2013 has a 90-hp advantage over the Marathon Seneca 12 2012's 10-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 Marathon Grumman Journey 2085FP 2013 is rated for 12 passengers, while the Marathon Seneca 12 2012 caps at 3. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Marathon Grumman Journey 2085FP 2013 could be the deciding factor.
Bottom line: Choose the Marathon Grumman Journey 2085FP 2013 if your priority is putting more people on the water - it handles 12 passengers and at 20,0 ft it has the deck room to back that rating up comfortably. The Marathon Seneca 12 2012 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 3 that costs less to run day-to-day.