According to the Oxford dictionary, shit is defined as "(i) Faeces, (ii) Something worthless; rubbish; nonsense", whereas crap is defined as "(i) Something of extremely poor quality, (ii) Excrement". In addition, to me as a non-native speaker, those carry the same meaning when used in the following sentences:
This book is crap.
This book is shit.
While neither are to be used in polite conversation ("This book is worthless" would be more appropriate), which of the two words is more rude?