really+to+do还是doing

都有可能。
really to do 是非谓语动词，属于附加成分；really doing 有两种可能：一是进行时态（be really doing，一是充当附加成分的非谓语动词。

扩展阅读：help sb do还是doing ... really后面加do还是doing ... reallytodo还是doing ... really后面接什么形式 ... computer ... hardly后加do还是doing ... really放在be动词前还是后 ... really放在冠词前还是后 ... really用在a前还是后 ...