You certainly need at least one of $a,b$ to be even to have any hope that one of these is always prime. However, even then it's not true. For example, take $a=38,b=143$, so $ab=5434$. neither $5433$ nor $5435$ is prime.
 As Arthur points out my example is rather larger than necessary. $ab$ must be a product of at least four distinct primes. If it is a product of more than four primes, it must be at least $2310$ (which isn't a counterexample). So we can look at the values in this sequence below $2310$ and check whether the neighbouring numbers are composite to find the first (i.e. smallest $ab$) few counterexamples. They are:
714, 870, 1155, 1190, 1254, 1330, 1365, 1518, 1590, 1770, 1785, 1794, 1806, 1938, 1995, 2046, 2145, 2170, 2190, 2210, 2226, 2262