The difference is that as bad as the F-35 is, cancelling it would have huge repercussions. On the other hand, the SLS could be cancelled tomorrow and nobody outside of NASA and the contractors would even notice.
Comparatively the F-35 is a lot better anyway. Its nowhere near as bad as people like to scream about it, it was just built for a world that doesn't really exist anymore and didn't meet the ambitious targets set for it originally. New F-16s are just as expensive (flyaway, presumably cheaper to maintain, but an F-16 is going to be a sitting duck against a stealth fighter with a modern sensor package as time goes on)
