We conducted a global survey on how men would like to be treated and most guys said that they preferred a girl be direct and honest if she wasn't interested.
We can't all like the same person, and I'm sure the guys don't expect you to date both of them at the same time.
So be polite, complimentary and honest. The quicker the better, the worst thing you can do is play both of them and get his hopes up. It's a surefire way to end up with neither of them.