Some people will say that " due to" must always be used when following some form of the verb "to be". For example, I was late due to heavy traffic. The start is delayed due to bad weather. I real language usage, it makes no difference which you use. I'm a native speaker and would use either in the above examples, in fact, I'd probably use " because of" most of the time. I'm sure you'll get a grammarian giving the subtleties involved, but if you are looking at general conversation socially, use either.