Reason for buying a 'standard' slow cooker over a modern electric pressure cooker with slow cooker option would mainly be cost.
The new electric/electronically controlled pressure cookers with slow-cooker option have so many safety measures built in that it would be extremely hard (almost impossible) to make them blow-up in pressure-cooker mode AND, I believe, are well worth the extra money if budget will allow. Also, one still gets slow-cook mode if one really insists on simmering a meal all-day, but pressure-cook mode will allow similar results in much shorter time. (Get home from work, chop food, click on the pressure cooker and, more often than not, pot-meal will be ready by time you have relaxed/changed clothes and sorted the pets out !) I paid over £100 for my electric pressure cooker, but cheaper options readily available. (My ex- luckily found a very well-recognised brand, pretty-much un-used, in charity shop for a few £s which works absolutely fine !)
Another reason to pressure-cook rather than slow-cook: if ya like your beans (in particular), but have a sensitive tummy, natural lectins can be a problem. As I understand it, slow cooking will not cook-off/neutralise bean lectins (red kidney bean being the worst I believe), BUT a pressure cooker will/should neutralise because cooks at much higher temperature.
Obviously not possible to cook a nice rice pud' and a curry or stew in the same pot at the same time. So a modern electric pressure cooker AND an electric slow-cooker would perhaps be a good combo if kitchen space permits.
