Another kitchen gadget to play with.

1. If you want meals without a lot of planning ahead.
Here's the biggest difference between a slow cooker and a pressure cooker: one is slow, and one is fast. With a pressure cooker, you can be unprepared until 6:00pm, when you come home with a nice cut of beef for braised stew, and still eat delicious tender meat at 7:00pm. With a slow cooker, no such luck.

So this is really a question of cooking style more than anything; do you like to set everything up before you go to work? Or would you rather not deal with cooking until you come home? WannaBeBeachBum says, "It's been a godsend when I need to get dinner on the table quickly, and the meal I want is more of a long cooking one. So basically when I'm disorganized, which is often."
2. If you're vegetarian or if you eat a lot of beans.
My experience has been that the pressure cooker deals with vegetables much better than the slow cooker, which tends to make them mushy. Also, I quit cooking beans in the slow cooker a long time ago because I found they fell apart and didn't taste as good as when I made them in the oven. The pressure cooker, on the other hand — wow. It cooks beans so fast, and without splitting them.

the polish chick says, for instance, "It really does depend on what you eat. we eat a tonne of beans, being mostly (80%) vegetarian, although my husband has made his amazing chicken stock in the pressure cooker and it was stellar. In fact, when we first got it and started to read about all the things we could [make] with it, we started to think that pretty soon we'd get rid of everything BUT the pressure cooker."

3. If you are tight on space in the kitchen.
A stovetop pressure cooker is also just a really big pot that can be used for many other things. A slow cooker insert, on the other hand, usually shouldn't be used for anything other than slow cooking. Also, while browning of ingredients is important for flavor in many slow cooker and pressure cooker recipes, a pressure cooker lets you brown and sauté right in the pressure cooker itself.

canada248 says, "The pressure cooker, as others have pointed out, doesn't have to just be a pressure cooker - don't tamp down the lid and it serves as a really big pot."

4. If you want to can a wide range of foods.
Some (but not all) pressure cookers are also pressure canners, which bring the temperature up higher so you can preserve lower-acid foods like soups. This is not something you could ever do in a slow cooker or regular water bath canning process. PreserveNation says, "If you think you might be interested in canning soups, meats or vegetables, then a pressure canner would be the better investment."

5. If you live at high altitude.
Cooking at high altitude can be tricky, since the boiling temperature of water goes down the higher up you go. A pressure cooker can be a big help. As lazy_lurker says, "I live at high altitude (Utah), and without the pressure cooker, beans (even lentils) will never get cooked."