Second the motion - get 2*16 and make it easy when you want to jump to 48 GiB or 64 GiB. (Note that I said "when", not "if" .)
Synthetic memory tests can show a performance difference with the number of DIMMs, but the big caches on the Xeons mean that most applications see little difference.
As a rule, For better performance, You generally want to utilize all four memory slots. The best ( And most expensive ) config would be to purchase four 8gb sticks of Ram. Apple Ram would set you back $800