On your save Vs xxxxx.

So wall of fire, for example, is based on save Vs spells.

Plague and poison are a bit different and require a check against con. Which makes them better for those elf Paladins with the faith virtue, since beating their virtue and their saves is tough, whereas you can easily beat the con check if you get past the virtue.

Bard songs are largely save Vs mental.

However, mental stats also come into saves a bit. For bard songs they can make quite a distance. For other stuff, less so. But high mental stats on the caster can also help in some circumstances.

Personally, I'd say just get your saves to a decent point, try to keep your con maxed, and otherwise don't worry too much about it, because the people that can exploit the rest are relatively few.

There's also some cabal powers that help a lot. A scarab power used to guarantee making all saves successfully, and a Scion power gives a chance to reflect stuff back on the caster if they aren't already in battle and it's a targeted spell or supplication.