On site is always best for clients especially for 600 people. It is easier than transporting and it serves as good marketing.
For those size parties you are potentially going to need a big smoker. May want to consider an Ole Hickory or FEC models. I cannot believe I just said that since I am a stick burner, but business is business.
However, I would probably go this direction if were doing 600+
stickburners do rule, lol!