outlook HTTP VS. TCP/IP

I have been told 2 different answers to is ee should check the box for HTTP of TCP/IP in the outlook profile settings? I had a user yesterday that is a remote user and he did not have the HTTP check box enabled and was having issues. He also had the cache mode box checked. I unchecked that but felt maybe he needed the HTTP VS. The TCP/IP check box and it did seem to clear it up. Which is the better setting to use for all users when  we look at the properties of the outlook settings? We use windows XP and office 2007. We do have VPN as well. I just need to know which is the correct way. Hoping an exchange guru can eaplain the best way.