If you build a good -7 I think a year from now you're gonna be wishing you built a -14. There are as many reasons for doing this stuff as guys using them, but in general, the monocular -14 is better suited to most of our uses. It's not the all-seeing eye of mordor; even the best gen III tubes won't magically allow you to see everything. The best set up is to have one aided eye, and one unaided. That way you are more aware of your surroundings, and also have some augmented vision. It's easy to get sucked into the image in the tube and not be aware of your surroundings. This may be as simple as walking into trees, stumbling on rocks, etc. or not realizing you are back lit and visible to others.
I'm not sure why guys build -7's, unless you really need to save that littler bit, or just want the retro look for whatever. The only real savings is buying a used device, which is chancy if you don't know what you're looking at. A good used -14 will be in the same price range as your -7 (or even -14) builds. But again, you need to know what you're doing. A -14 build kit will be around 2K, depending on tube. In fact, I think the housings and tubes for -14's are basically the same price as the -7's, especially since a lot of guys upgrade to a gen III tube.
So again it's a matter of which format works better for you. If you plan on being mostly stationary, star-gazing, etc. then the -7's will work. But if you plan on moving around with them, as in hunting or patrolling, then the -14's shine.
Yeah , I know, someone is gonna say, but Diz, I've been watching the new Seal Team show and those dudes all have the new binos, so isn't that format better? Well, for them, sure. They have the latest tech in there which is a far cry from the -7's, and they have a whole team to cover multiple angles. So unless you are sporting the latest hardware, and running with a whole team, you are better off with a monocular NVD.