I suppose it really depends on how comfortable/familiar they are with firearms. A lot of females might not like a handgun because their first comment upon picking up a polymer pistol is how heavy it is, where men would probably say a glock is light, especially when comparing it to a steel 1911. Women (especially older women) will more than likely have an issue with racking the slide on a semiautomatic pistol. They will most likely use the over the top method, which can also result in pinched fingers. I think a revolver would be pointless because they almost all shoot ridiculously overpowered magnum loads. So for a female I think anything in the AR platform would be the way to go. An AR15 is extremely shootable in any caliber due to the recoil being straight back into your shoulder, and causing little or no muzzle rise. I have a hi point 9mm carbine, and although I like it, I would much rather shoot my AR in 223 or 300 blackout. Even though 9mm is a pretty light cartridge, the hi point doesn't deliver the recoil very well. ARs also have adjustable stocks for smaller people. The only issue with the AR is the price.
For the guy I think he would take to a handgun much more. It can be kept in a nightstand for easy access, and if he was in the service or law enforcement he should be familiar with handguns and be able to re-learn how to shoot one very quickly. But if it's a 1 gun for both of them deal, some sort of carbine is the way to go. I don't want to exclude things that aren't an AR because I haven't seen or shot any carbines from any other manufacturers.
Overall the thing with it being a "home defense" gun, is all in the head of the owners. Unless they live in a really bad neighborhood, the chance of a break-in is low, and the chance of somebody breaking in with the intent to harm the occupants is even lower. If somebody breaks in they should retreat and lock themselves in the bedroom with a loaded gun pointed at the door on the phone with the police. So the biggest thing is if they are ever in that situation, give them a gun that they feel completely confident they will be able to defend their life with. Get them something that has has manageable recoil so they will want to practice and get good with it.
Another option is always a Ruger 10/22. It is absurdly easy and fun to shoot (as long as you can find ammo), and it's not intimidating for anybody to learn to shoot. And although it's just a dinky little 22, it is enough to get the job done (especially when you have 25 shots). If you can't hit what you're aiming at and recoil makes follow-up shots difficult, then the gun isn't suitable for home defence. An intruder is going to be more deterred by bullets entering his body then by a person unloading a noise machine and not hitting anything. This is just my opinion, as I have never been an intruder and have never been shot at.
and the carbine makes it easy to shoot the 10mm accurately and with very soft recoil.
