I don't like the 3-gauge consoles very much because the end of a console is a very inconvenient place to put your compass, you really want it directly in front of your face, so wrist-mounted is ideal for that. If you wrist-mount your computer as well, now you're down to just a 1-gauge console (SPG).
For the redundant analog gauges: say you are on a dive and your computer fails. Damn, but at least you've got your backup analog gauges. Now what what would you do with the information on the gauges? Unless you've been using tables before every dive, you won't be able to figure out your NDL on the fly. For many divers, the plan to deal with a failed computer is to simply end the dive as soon as you realize the computer is dead. This isn't as bad as it might sound, because computers don't fail very often. The real upside of backup analog gauges is for the situation where you're trying to do many dives, many days in a row, say on a live-aboard or a dive resort. If your computer fails there, you'll be done diving for that day, but can continue diving the next day on the analog gauges and tables. A backup computer would achieve much the same, although it's more expensive of course.
Air integration on a wrist-mounted computer is cool because then you don't need an SPG hose at all. But you pay a lot for that convenience -- the computer will be more expensive to begin with, and wireless transmitters themselves run about $400. If I were on the market for a recreational dive computer, I'd be looking at the Deep6 Excursion ($250 when you buy two) and the new Shearwater Peregrine ($450 each). Both are very solid machines that should last you a really long time. If budget is the main concern, go for the D6. If you're gonna splurge on your computer, I'd say go for the Shearwater over air integration any day of the week.
As for whether your son needs one, well, it kinda depends on your kid. If he has one, understands how to use it, and actually does use it, then it will increase his safety. Both by giving him the information it provides, and by making him less dependent on you. If he doesn't have one or doesn't use it properly, then it's just gonna make your wallet that much thinner. Of course if he's unable to do that, well, maybe he's not quite ready for diving
. I'd say do it.