Prevalence of rhotics in English and IPA transcription

Any suggestions on the best way to transcribe English rhotics in the IPA? I especially use transcription in comparative contexts with French, so I do need to distinguish between the French and English phonemes, but I don't care about allophones as this is strictly phonemic transcription (for ESL students).
I'm not using the right-side-up 'r' because too many languages use the trilled rhotic that it represents. I'm currently using the symbol for an alveolar approximant (upside-down, lowercase 'r'), but I'm wondering how widespread this rhotic really is in English. Can someone enlighten me? How does it compare in prevalence to the retroflex approximant? I think the latter may be more common but the symbol for it in the IPA is harder for students to recognize, and as long as both are allophones of the same phoneme, I may as well use the upside-down 'r'. My own (standard American) pronunciation seems to use the retroflex allophone.

Similarly, I'm wondering about the best symbol for French. The small uppercase 'R' represents a uvular trill and is easy to recognize, but standard French doesn't trill its 'r'; it is just a fricative (which in theory requires an upside-down, small, uppercase R). I think the trill symbol is okay because these two sounds are also allophones of the same phoneme, but opinions are welcome.

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