If I recall properly, when the system was changed by Zulg some years ago, resistances became their own specific category within the game. That means that they act independently of saves. If you're resist disease then you'll be affected less from said disease if you catch it than someone who isn't, both by damage and physical affect. If you're resist to elements, elemental damage will hurt you less, etc.

So, if you make a save on an elemental spell that's cast at you, the damage could be lowered or you could resist it's secondary affect, like chill touch and you could have a similar affect if you have resist but it couldn't be negated completely. Without seeing the coding, I'd guess that each spell has a number of factors that it checks for landing and effectiveness and each plays a role. That would mean that every spell which has a different set of metrics that it checks when cast, would differ in how each works.

In your example, without me looking at code, a save could stop the plague from landing because that's what saves are for, while resist could only help with that if it's built into the spell as a check but would help with any lingering affects or damage.

Regardless of what it does, a resist to any incoming attack is helpful.