The spec sheets list the Malibu Response LXi 2007 as modified vee and the Malibu Sunscape 21 LSV 2008 as v25 diamond, so check how each hull rides in the conditions you use before you decide.
On paper these two are close siblings in the size department - Malibu Response LXi 2007 at 20,0 ft versus Malibu Sunscape 21 LSV 2008 at 21,0 ft. Weight tells a clearer story for trailering families: the Malibu Sunscape 21 LSV 2008 tips the scales at 3 500 lbs - 700 lbs less than the Malibu Response LXi 2007 at 2 800 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 - 400 hp for the Malibu Response LXi 2007 and 400 hp for the Malibu Sunscape 21 LSV 2008. 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. Fuel capacity breaks the other way: the Malibu Sunscape 21 LSV 2008 carries 46 gallons versus 41 gallons in the Malibu Response LXi 2007. On a lake day that's negligible, but for coastal cruising or long reservoir runs the extra range matters.
For family outings this is probably the sharpest distinction between the two. The Malibu Sunscape 21 LSV 2008 is rated for 12 passengers, while the Malibu Response LXi 2007 caps at 8. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Malibu Sunscape 21 LSV 2008 could be the deciding factor.
Bottom line: Choose the Malibu Sunscape 21 LSV 2008 if your priority is putting more people on the water - it handles 12 passengers and at 21,0 ft it has the deck room to back that rating up comfortably. The Malibu Response LXi 2007 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 8 that costs less to run day-to-day.