in hindsight, the biggest measurable differences are twofold.

1) trump has been terrible on the climate. clinton may not have been very good, but trump is clearly worse. i think that much is not controversial.
2) trump has been better on foreign policy, clearly. it wasn't clear how he'd govern - he said a lot of things that didn't make sense. but, he's largely withdrawn from syria, which is good. and, he's mostly avoided starting new wars anywhere. clinton was crystal clear that she would have been far more aggressive in using force.

obviously, you want an option that is better than either - you want a better climate policy than trump's, and a better foreign policy than clinton's. but, that didn't exist, or at least not in the form of a major candidate.

and, if we're debating which is worse for the planet in the long run, it's not an easy answer; they're both bad, granted, but somebody has to govern.