Buck converter or a linear regulator for a low current 3.3V source
At 125 mA, a linear regulator from 5 V to 3.3 V wastes (5 V - 3.3 V) · 125 mA = 0.22 W, which is not much compared to the 25 W being delivered to the Pi cluster.

If you used a buck regulator instead, you could probably get 90% efficiency without too much effort, which corresponds to a waste of (3.3 V · 125 mA · 10%) = 0.04 W.

However, a buck converter will be somewhat more complicated, somewhat more expensive, and will likely have a noisier output. It really depends on your specific application whether saving 0.18 W is worth the extra complexity.