+1 again for riding both. I rode spd for about 15 years before deciding to try flats. Make sure you get decent pedals and shoes though or it will put you off (nanos and 5-10s for me - very grippy). It took about 2-3 months to get used to riding them properly but it certainly helps technique. As above, forces you to get your heels down more, push forward and move with the bike rather than lift with the feet. It takes some getting used to but I became a quicker/smoother rider as a result.
I've switched back to spd recently as we tend to do mostly xc stuff over the winter and pedal position can be trickier in the slop. Switching back isn't too difficult - just move your cleats right back and use the same heel down technique. Climbing is much easier with spd and I found they can make you more committed to a tricky section rather than thinking of dabbing.
I'll probably go back to flats again after the winter.