I study French and German at GCSE and in short, French is the easier language, so if thats your main consideration, then pick French. However, it is perfectly possible to get A*, even full UMS, if you put some work into them. Therefore, I would suggest picking the language you prefer the sound of, or whichever feels more natural to you. After all, you will be studying it for 2 years, so why not enjoy it?!
All the best with your choices and GCSEs!