The Xpress HD18 2011 is a flat-bottom design and the Xpress SF Series - X20SF 2008 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.
The Xpress SF Series - X20SF 2008 tops out at 200 hp. Engine specs for the Xpress HD18 2011 aren't listed - confirm with a dealer before selecting an outboard.
For family outings this is probably the sharpest distinction between the two. The Xpress SF Series - X20SF 2008 is rated for 6 passengers, while the Xpress HD18 2011 caps at 5. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Xpress SF Series - X20SF 2008 could be the deciding factor.
Bottom line: Choose the Xpress SF Series - X20SF 2008 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 Xpress HD18 2011 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 5 that costs less to run day-to-day.