The differences between shell types are mostly cosmetic. One shell type isn’t harder to clean or more slippery than another.
The interior shell is the main visual focus when shopping for a hot tub in a showroom, but keep in mind that once it’s installed, the shell itself is almost completely covered with water and the perimeter shelf can only be seen when the hot tub is in use, which is often after dark anyways.
The only time you’ll see the shell is when you’re actually in it. Otherwise, the hot tub will almost always be seen with the cover on, which means the cover and skirting combination are far more important than the shell’s color and pattern.