Canonical tag SEO - Expert views need

Keep the one that's ranking better and redirect the other URL.

No need to overthink this.
 
Nargil pretty much said everything.
I just want to add that you could merge those 2. Keep the ranking article and then use the second article to add more info to the ranking one. Hope, it makes sense.
 
There is no problem and you can add the canonical tag for the same webpage for preventing your webpage content from copying. One more and important thing, as you said you have the same topic but the content is different, so don't use the same keywords for both pages as it will cause keyword cannibalization and it will hurt your Search Rankings.
 
Back
Top