The only time that script language makes a difference is if you're going to be creating what are called "Connected Applications" where TestComplete script routines/code is called from within the application under test. In those cases, the script language you use should be the same as the application under test.
If you're not creating Connected Applications, then the only criteria that I would say would make a difference in scripting language is personal familiarity. If you're familiar with VB, then VBScript would probably be easiest for you to pick up. If you're a Java guy, then go with JScript. I personally know Delphi/Pascal very well so that's my primary default but I'm teaching myself JScript as I go simply to expand my ability to assist others in different languages.