I have one bike set up with SPD PD-M324 pedals and one with SPD-SL PD-6620 pedals. I use Specialized Tahoe mountain bike shoes with the SPD pedals and Specialized BG Elite Road shoes with the SPD-SL pedals. Note that the road shoes would also accept SPD cleats.

My experience is that my feet get numb when I ride more than 20 miles with the the mountain bike shoes on the SPD pedals. I assume the soles are just not stiff enough and the SPD cleat are is small, so I get a lot of pressure in one spot. This is not a problem with the stiffer road shoes with the road pedals. But for short rides, the SPD pedals are fine, and the MTB shoes are much easier to walk in.

Since your rides on the sprint triathlons are probably about 12 miles, either should work for you.