I was considering both, and ended up with the Flame Mahogany based on looks. I'm a big fan of terrific looking wood and although spruce is very good acoustically, it's kinda boring looking. Besides, I have a Lanikai spruce top tenor (love it) and the spruce travel soprano would look too similar for my tastes.
I considered the size of the travel uke, but figgered that a soprano is pretty travel sized already.
Ive been very happy with my choice.