Under the usual convention of 95% significance, it's neither :-D

 > binom.test(508634, 1000000, 0.51) Exact binomial test
data: 508634 and 1e+06
number of successes = 508634, number of trials = 1e+06, p-value = 0.006304
alternative hypothesis: true probability of success is not equal to 0.51
95 percent confidence interval:
0.5077 0.5096
sample estimates:
probability of success 0.5086