Underwood 9mm +p+ 124gr = 1,298fps
New Underwood 357 sig 125gr = 1,475fps (as stated by Underwood on GT, not tested yet by TNOutdoors9)
The rounds tested by TNOutdoors9 for 357 sig are all loads from the bigger guys that don't make hotter 357sig loads(Federal, Remington, Speer, hornady).
Let's compare apples to apples.
Having said all of that, the 9mm is a fine cartridge and would serve you well.