Differential Diagnosis Achilles vs Retrocalcaneal Bursitis

Hi this is more out of interest and devolopment then any actual patient that I have. Just wondering how people go about differntially diagnosis for these injuries and how you would change your treatment towards the two injuries. I have also read that retrocalc responds reasonable well to coritzone injection compared with tendonapthy which increases your chance of achilles rupture. I realise that you would not refer for this until other Rx has failed but just wondering peoples thoughts on this as well.
Thanks