The 3.0 makes more since for most people. I'm not sure how to 'justify' paying the extra 10k, unless your driving style really needs it. The highways here are so congested etc, it's hard to find any real rationale for it regardless of style. But for some, maybe it's just a prestige or mental thing knowing they have the V8.

Also, I consider a 10% difference in MPG to be significant. Over the course of time, it will save quite a bit of additional money. I'm not sure why people don't seem to think it's a big deal. I bet that if there was a gas station in the area that was 10% cheaper than the other ones, (e.g. $3.15 versus $3.50), people would line up around the corner to go there.

Again, I would not use efficiency as my main consideration, but I do think one should think it through when making the choice.