6-7 knots is "no-wake"? For my boat, a SD hull, a no-wake speed is 4.5 kts or less. Anything higher and I am putting out a noticeable wake. There is a reason it takes me 20 minutes to get out my harbor, all of which is a no-wake zone.
The the OP, I think the best advice is what Mark mentioned earlier. Why are you out boating? If it is to get to another location, then a faster boat is very nice as it allows you to get there quicker to enjoy that location. If you are boating for the experience of getting to another location, then a displacement speed is just fine.
Of course for all of us, we are somewhere in between those two extremes. That may be why there are so many of us that own SD hulls instead of FD or planing hulls. An SD hull is a compromise. There are also a lot of differences in those boats. Mine for example, will cruise very comfortably at 7-8 kts. 7 kts at 2 gph is pretty economical. I can go 10-11 kts but it is very loud and burns 5x the fuel. I am happy at 7 knots. Others have SD hulls that can get up to 15 kts without too much headache and still be very economical at 8 kts.
So have you and your wife figure out where on that spectrum from "hurry and get to where I want to be" to "I'm where I want to be on my boat at 6 kts" you think you fall. Depending on where that is, pick the boat that will match.
I also agree with those that have mentioned that some of the blue-water FD boats out there are so expensive that you will never make up the difference in fuel efficiency. However, there are many reasons to buy a boat such as a Nordhavens, Selene, KK, etc... Some like the cachet of owning a passage maker. Others just like the looks (face it, we all pick boats partly because of their looks). The thought that your boat can handle most any weather you might run into is another comforting thought. Honestly though, most owners of those boats don't take full advantage of their capabilities.
Many of us on TF own, or have owed, sports cars. I have a poor man's sports car, a 350Z. Great car! However, it makes no sense to own it. I haven't raced it in a few years now. I rarely have access to roads where its speed, acceleration, and cornering abilities give me any advantage over my old SUV. Most of the time, the poor thing is relegated to being a daily driver. Yeah, it is a fun daily driver, but a daily driver none the less. Boats are the same. If a passage-maker catches your fancy and you can afford it, go for it. Just realize that the excuse of fuel efficiency is just that, an excuse.