I don't have experience with ultrasonic but I do have experience of both poison and cats and I can tell you they are BOTH nasty.
What you need to do is to get a pest control specialist in and get them to investigate, fully, the places in which they are able to get into your home. My experiences:
Many mice in a flat. LL got a firm to put down poison. It was awful. Not only did it take months to take effect (they had to keep coming back), but the poor sods slowly died somewhere and in the meantime left trails of piss and blood all over my flat, because even after they started blocking up what they thought were the entry points, it was too late.
Mice can climb walls (and iirc up the interiors of drain pipes) and get through a hole the size of a pencil tip. That's why you need a specialist to find the entry points.
Cats are great at catching mice, but they'll bring you them, play with them, half maul them to death, then present them to you as a 'gift', and you then have to 'dispatch' them - because they very quickly get bored with killing them. And it's awful and it makes you feel bad.
So, that's my recommendation. If the ultrasonic is cheap, go for it, but you have to find the entry points. They need blocking up with - at the very least - wire wool.