Personally, I would choose Red Top (or Army Corps), because of its' natural setting and all. But the 4th of July will be a busy weekend I expect, so you may want to make a decision and get a spot wherever you choose.
Also, you have the town of Woodstock near you (at Red Top) and they always have stuff going on that weekend - kind of a cute little town/square. Also second Marietta Square - some great history and awesome restaurants, and you don't need to lose an arm to eat there. A lot of places have outside seating so assuming the weather is accommodating that makes it even nicer.
And if either of you are runners the Peachtree Road Race (10K) is held in ATL on the morning of the 4th. This is a great event, although a (nice) zoo because there are 55,000 runners. You need a bib, so check out online in Feb/Mar.
And on the 4th of July in ATL, if you are looking for fireworks, you may only need to stick your head out of your trailer.