Who would you ask for a recommendation?

I would say go with recommender A, unless his English is egregiously bad. I faced a similar problem when putting together a college rec. I had worked in a lab in hs and my direct supervisor we from Serbia and not a native English speaker. I considered asking another person in the lab who I had worked on one small project with to write my rec but in the end decided to go with my direct supervisor who I done my most important project with because I felt like he could speak more about the quality of my work.

I feel like you should choose someone who knows you and your work to write your rec. This way they can cite specific examples to answer the questions on the apps. They probably also have more to say about your personality and work ethic than someone who doesn't know you as well. I don't think minor grammatical mistakes will take anything away from a good recommendation. The adcoms should keep in mind that not everyone asks native English speakers to write their recs. This is America after all and there are plently of very capable people are immigrants and don't speak the language perfectly. As long as his writing is understandable and there are no distracting mistakes I'd say go with someone who knows you better.