It depends. If it's a
sign that is meant to be there for a long time and is of a very professional nature then 6mm but it's heavy. You can also use Alumalite which is light but you should use an edge cap for a more finished look. If it's a work site
sign screwed to 2x4's, a
sign at a baseball field advertising a sponsor, then 3mm.
There is a cost difference between Alumalite and 6mm. Sometimes the customer doesn't care about the weight so in that case just use the 6mm. If they do and they want a nice finished look use the Alumalite and the plastic edge cap. I once made an oval
sign from Alumalite and a flexible edge cap. It still looks great after 8 years! The edge cap is very cheap and easy to apply with a little silicone to hold it in place, Liquid Nails or whatever.