Hot vs. Cool

You can see there's a hot vs. cool argument happening here, where hot is Louis Armstrong while cool is Bix Beiderbecke.

Note

You can literally think of hot as being spicy, sharp, etc. while cool is more mellow, refreshing, calming.

Notice this in Armstrong's When You're Smiling piece, where:

It's a slower, cooler piece, in contrast to a lot of his other pieces. He gets to scat a bit more here, but even here when he gets to his solo in the middle we see more of his register and range. It's easy to see how it contrasts with the slower nature of the piece.
Notice the vibrato on the high notes too.
Let's listen, in contrast, to Beiderbeck's Riverboat Shuffle, where:
It's more up-tempo, like an Armstrong piece.
But there more mixed soloing (again some of that organized chaos in comping from Ragtime)
Even the solos are more tame. They have less register range than say Armstrong's.
If people do solo, it's usually Trading 4's or something smaller.