Updated: March 26, 2018
If you had to choose just one you would want Google Analytics.
Because it's the industry standard so if you plan on applying for an ad network or wish to sell your blog in the future, people will ask for your Google Analytics numbers, not your WordPress ones.
Plus, many bloggers notice that there can be a discrepancy between the WordPress stats and the Google Analytics stats.
Personally, I don't bother with WordPress numbers. I have Google Analytics installed on all of my blogs and use the Google Analytics mobile app to keep track of things on the go.
Plus, what happens if you move platforms? Maybe you don't want to be on WordPress forever and you switch to Squarespace. If that's the case then when you move you'll lose all of your stats!
That would be terrible and you don't want that. I don't want that.