I have a 2006 X45 with the 8.1. To give you an idea on fuel consumption, when we are Lake Powell we spend a lot of time each day out on the lake doing all kinds of playing: surfing, wakeboarding, kneeboarding, canyon exploring, etc. We are probably on the lake 8 or 9 hours a day and go through 40 - 50 gallons of gas per day. I don't know how this compares to the LY6. However, there are a lot of reasons to want the extra power of the big block. The X45 is a large heavy boat. If you load it up with people and/or fill the ballast tanks you'll want the extra power, especially the low end torq for surfing or pulling out a skiier. If you ever use the boat at elevation you'll also want the bigger engine. I'm from Utah and for many years the local MC dealer wouldn't bring in an X45 without the 8.1 due to elevation. We're at about 4000 ft but many of the lakes are higher.
Given that the 2008 has 25% fewer hours, more options, the bigger engine, and your wife likes the color better, I'd say you'd want to give it a pretty close look!