The spec sheets list the Malibu Response LXi 2008 as cut diamond and the Malibu Sportster 2005 as modified vee, 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 2008 at 20,0 ft versus Malibu Sportster 2005 at 20,0 ft. Weight tells a clearer story for trailering families: the Malibu Response LXi 2008 tips the scales at 2 800 lbs - 550 lbs more than the Malibu Sportster 2005 at 2 250 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 2008 and 400 hp for the Malibu Sportster 2005. 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. Both carry nearly identical fuel loads - 41 gal and 38 gal - so range won't be a tiebreaker here.
For family outings this is probably the sharpest distinction between the two. The Malibu Response LXi 2008 is rated for 8 passengers, while the Malibu Sportster 2005 caps at 6. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Malibu Response LXi 2008 could be the deciding factor.
Bottom line: Choose the Malibu Response LXi 2008 if your priority is putting more people on the water - it handles 8 passengers and at 20,0 ft it has the deck room to back that rating up comfortably. The Malibu Sportster 2005 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 6 that costs less to run day-to-day.