Although US and AA are both in Terminal B at BOS, they are in different parts of the terminal, so, unless there is a shortcut that I am not aware of, you will need to exit the AA part of the terminal and cross a small parking area to reach the US gates, where you will have to re-clear security. Once you're through, the US club is quite nice -- big, with a nice view, and rarely crowded. Terminal E is not remotely close by (you would have to take a shuttle bus to get there), so you will certainly want to stick with the US club, I think.