Neither one is better from a language point of view since they are the same, array indexing should just be syntactic sugar for pointer arithmetic. We can see this from the draft C99 standard section 6.5.2.1 Array subscripting which says (emphasis mine):
[...]The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))).[..]
Although for others reading your code a[i] is probably more readable.
