zeros(Int64, n) does initialize the Vector with zeros, Int64[]; resize!(x,n) only ask for the memory (if you print it before overwriting you will see garbage). Probably just for the setup the resize! solution is faster.
The difference in access performance you are seeing probably comes from zeros having already materialized and brought the Vector to the L1 cache, while accessing the positions in the Vector expanded by resize will cause a page fault (i.e., the OS was lazily waiting for you to access any position to only then really allocate the memory), and then some cache misses of bringing the Vector slowly into the L1 cache.