I also offer both Shellac and ACG.
There are pros and cons with both systems...
Shellac removes much easier imho.
ACG has more of a choice of colours and I think it's easier to apply, especially if Shellac has gone a little thick.
Some of the ACG colours are a little too sheer and require 3 coats, there are't many Shellac colours that are like that, they appear more heavily pigmented.
Some of my clients LOVE Shellac and some LOVE ACG, it's horses for courses but I'm happy being able to offer both systems.