I always choose Mach and am not sure why.
The only time I used the Acro Bike was to get into Sky Pillar.
You mean the other way around? We need Mach Bike to get to Rayquaza in Sky Pillar. I always chose Acro Bike because I thought the name sounded cooler, after which it became a habit. If I had to permanently choose one, it will be Mach Bike because of the mudslides and getting Rayquaza.