We have stayed at both places while also visiting our son and family in Brentwood. Poole Knobs is an older campground with smaller/hilly sites. Getting into several of them with our 43' MH is impossible without dragging severly. Our first trip there, we had a site that was up a steep grade and I wasn't going to tear up my rig, but there was a site that I would fit on that was available and I was able to change. Bottom line, I wouldn't make a reservation on a site there without knowing for sure that you would fit and get into it. Having said that, there were a few sites that that were good for a big rig.
We now prefer Seven Points because there are numerous sites that we can fit on and are easy to get into. As far as closeness to Brentwood, I feel that both campgrounds are about the same time line to get there.