Best aspect ratio for external monitor

I'm looking at Dell Ultrasharp 24" monitor for my MBP 13" late 2011 (ElCapitan). There are 2 models with 16:10 (U2415) and 16:9 (U2417H) aspect ratio. Which aspect ratio works best with MBP? If using the 16:9, will it stretch the display when mirroring or extending MBP display?

Thank you.