If by cooling you mean VRM cooling, then the b550-m does. It also has a better power delivery, meaning more stable overclocks, and just overall better experience. It's well reviewed for excellent VRM thermals.
Then again, the same goes for the b450, but at the takeaway of less power phases, which means (although you most likely won't feel a real life difference) that it'll be less stable at higher overclocks.
To conclude, the B550 has a better power design and PCIE gen 4 capabilities, and the B450 lacks those but it is cheaper, but has a larger form factor. This means that there are more expansion slots, so if you wanted to put say wifi cards in or internal capture cards, you would have the freedom to do that.