Rust really only affects sungrown arabica. We've got some rust on our own (shadegrown) trees, but if they're healthy and happy and not too hot, they stay ahead of the fungus pretty handily.

If you're just after caffeine, though, robusta has more caffeine than arabica.

The downside is that robusta needs pollinators and arabica doesn't - so you're damned either way as the century wears on. You'll probably need to learn to like excelsa.