I went with the 17.5 because I wanted higher velocity. Maybe I’m wrong about this, but I feel like it’s easier to tune and get better fuel economy.
Higher velocity means lower pressure. Lower pressure helps fuel vaporize.
If you want to build a max rpm machine, you need a big carb. If you want torque, go with a moderate size.
All that said, 1.5mm isn’t that much different. There’s more information on the tuning spreadsheet for the 19. If you don’t have a specific reason to go with the smaller one, like I did, then 19 is perfectly good.