Which is better to use class A or class B?

For public addresses, you use whatever is assigned to you. You probably won't be able to find available class A or class B networks, so you will use a class C network, or part of it. Soon you won't be able to get that, either (time to switch to IPv6).

For a private network, a class A network - 10.0.0.0 /8 - gives you the greatest flexibility, because of the larger address space.