They solve different problems, not the same one
It's common to think of these two appliances as "slow" and "fast" versions of the same thing. In reality, each one fits a different kind of routine and a different kind of result.
Slow cooker
Cooks at a low, steady temperature for 4 to 10 hours, ideal for setting up in the morning before heading out to work. The result is very tender textures in cheaper cuts of meat, stews that benefit from long cook times, and very little risk of anything burning while you're away.
- Pro: can be left unattended for hours, ideal for long work routines.
- Con: not fast — doesn't work if you need dinner in under a couple of hours.
Electric pressure cooker
Cooks at high pressure and temperature, drastically cutting down the time needed for tough cuts of meat, dried beans with no pre-soaking, or richly concentrated stock — from hours down to minutes.
- Pro: much faster, versatile (many models also sauté and cook rice).
- Con: requires more attention during the pressure-release process, and the initial learning curve is a bit steeper.
Is it worth having both?
Many current models combine both functions in a single appliance (a multi-cooker), which makes sense if your kitchen has limited space and your routine shifts week to week between both cooking styles.