Are the liners that came with my Prime good enough for salt, snow and grime (especially on the sides) or should I go with Weathertechs or something else?
I have the all-weather liners from Toyota in my prime and WeatherTech liners in my Gen3 Prius. I find the Toyota liners to be very high quality but you are correct that they don't have the same height on the sides. In my experience/usage, I don't seem to track a large amount of slush/snow into the car and don't usually see a large accumulation of liquid in my Gen3. Haven't been through a winter with the Prime yet but I believe the Toyota liners will be just fine.
I just don't want salt, etc to get caked on over time especially on the sides. I'll have to keep an eye on it.