Slow Cooker: I have used one for some years and find it very useful, although I'm usually around the house when it's on. It was first recommended to me by a hairdresser who used to go home at lunchtime to put hers on, ready for the evening meal, and I never heard of any problems. A slow cooker always has liquid as part of the recipe, so it shouldn't dry out, as it is covered by a lid. I've mostly cooked casseroles in mine, but you can do other things as well.
Pressure Cooker: I used one for years, a Prestige model, and it wouldn't need much liquid, but cooks very quickly, of course, and you need to be nearby while it is working. I never had problems with mine, but I don't use one now, as I find it quite heavy to lift. Vegetables tend to go a bit soggy if they are overcooked, but a pressure cooker is very convenient if you only want to cook on one ring. I also did many casseroles in mine, and I would say, the meat would be more tender in the pressure cooker, rather than the slow cooker. The pressure cooker makes a certain amount of noise, with hissing, which makes some people nervous, while the slow cooker makes no noise at all. In a power cut, you can still use a pressure cooker if you cook by gas, as I do, as the slow cooker would be no use at all!
It depends on your lifestyle whether you would use either, or both, and equally safe if treated correctly. If you know someone who owns either, ask if you can borrow it, and try them both out to see which would suit you best. I used to be an HE teacher, so that's the best advice I could give you.