From the top of my head what Swiftkey do better than WP Keyboard:
1. The text autocorrect is much better on other keyboards. Some words I feel should just auto correct and sometimes it should at least have the word as an option. This is especially on other languages like Swedish but even English sometimes I feel. I misstype much much more on my WP because of this.
2. No option to enable Vibration and turn on/off sound. I mean, this is just such a basic setting that should just be there.
3. On Swedish keyboard the "long press for a number" does not work at all.
4. No themes, not that it matters to me but I know some people find it important.
5. You can disable "flow" on SwiftKey to instead enable gestures like erase, capitalize and hide keyboard. I never use flow so that would be more useful for me.
Then there's of course things WP Keyboard do better than Swiftkey.
Edit: Just to give an example on how Swiftkey is better on autocorrect than WP.
I wrote "This is a test to see how good the autocorrect is on windows phone" very fast on the WP and the result was "This is a test ti se how fiod the autocorrect is on windows phone".
I have no idea what words it did autocorrect but it at least failed to autocorrect ti, se and fiod.
Then I wrote that exact sentence on Swiftkey and the result was: "This is a test to see how food the autocorrect is on windows phone". As you can see it managed to autocorrect ti and se correctly and at least tried to autocorrect fiod to food but it was wrong.
I know this is no way a complete test to compare them both but it gives you a pretty good idea on how much better SwiftKey is on predicting words.