I am not sure if there have been many large trials which compared cisplatin vs. carboplatin as sole radiosensitizers, but in clinical practice a number of centers now use carbo because they get equivalent results without as many noxious side effects. This article from Moffitt Center, which cites two trials:

Treatment of Locally Advanced Head and Neck Cancer: Historical and Critical Review

Muhyi Al-Sarraf, MD, Cancer Control 9(5):387-399, 2002.

CG to husband Barry, dx. 7/21/05, age 66, SCC rgt. tonsil, BOT, 2 nodes (stg. IV), HPV+, tonsillectomy, 7x carboplatin, 35x tomoTherapy IMRT w/ Ethyol @ Johns Hopkins, thru treatment 9/28/05, HPV vaccine trial 12/06-present. Looking good!