Only their colours are different; they don't perform any better or any worse than each other. So go ahead and choose whichever you want, because they both work the same way.
Guess Game Freak ditched the idea of the Mach Bike and Acro Bike from Gen 3, which is kinda unfortunate. Being able to switch between the two and go at different speeds on the Mach Bike was nice; but I guess the Acro Bike's gimmick is taken away when the Roller Skates are given to you.
