The spec sheets list the Malibu Response LXi 2008 as cut diamond and the Malibu Ride XTi 2006 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 Ride XTi 2006 at 21,0 ft. At 2 800 lbs and 2 900 lbs respectively, both sit in a similar weight class - either should pair comfortably with most mid-size SUVs and half-ton trucks, though always confirm your specific tow rating with the motor added.
Both boats share a closely matched power ceiling - 400 hp for the Malibu Response LXi 2008 and 400 hp for the Malibu Ride XTi 2006. 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 Response LXi 2008 carries 41 gallons versus 37 gallons in the Malibu Ride XTi 2006. 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 Ride XTi 2006 is rated for 10 passengers, while the Malibu Response LXi 2008 caps at 8. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Malibu Ride XTi 2006 could be the deciding factor.
Bottom line: Choose the Malibu Ride XTi 2006 if your priority is putting more people on the water - it handles 10 passengers and at 21,0 ft it has the deck room to back that rating up comfortably. The Malibu Response LXi 2008 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 8 that costs less to run day-to-day.