One difference is that sizeof(a)-1 will be replaced with the length of the string at compile time. With p you need to use strlen(p) to get the length at runtime. Also some compilers don't like char *p="string", they want const char *p="string" in which case the memory for "string" is read-only but the memory for a is not. Even if the compiler does not require the const declaration it's bad practice to modify the string pointed to by p (ie *p='a'). The pointer p can be changed to point to something else. With the array a, a new value has to be copied into the array (if it fits).
