Best Antenna for my Bobcat 300

Hi:

I have a Bobcat 300. I want to put it on the top of my house in Chicago. My house is 42 feet high with a direct line of site to downtown Chicago. I am about 6 miles from downtown. I would place the antenna on my roof about 50 feet high total or 15 meters.

Should I go with a 5.8 dbi antenna or 8 dbi?

Go here and get a free account to test different things out. This will give you a better idea of what you should try.