Res Con will guarantee your concentration saves against most hits. If you want to regularly utilize concentration spells on the frontline, then that's a must IMO. Also, it provides a defensive benefit toward Con saves - which are fairly common and can have some very annoying effects.
Warcaster helps on saves. It provides an OA benefit mainly to characters with Booming Blade (something you won't have staying straight paladin). But it's true benefit is if you have off turn spells like shield you could cast while still having your weapon drawn. Unless you multiclass you won't have those and so while it's a good feat, it's not as good as Res Con for you.
I think that's a wise plan.