As far as space you'll be fine because both are Premium loops so your trailer will fit on the site no problem.
I'm of the same opinion as luckycharm. But if someone held a gun to my head I would choose 400. The reason why is it's a smaller loop shaped like a lollipop (circle with a small stick). 700 is long and straight. Both are close to the Pioneer Hall/marina area. But here's the rub. Some of the 700 sites are, IMO, closer to PH than any of the 400 sites. On the flip, though, ALL of the 400 sites are probably closer to PH than the furthest 700 sites.
Can't go wrong with either one. Each has their plusses and minuses. Relax, don't stress over it, and enjoy.
Bama ED