The gist of the argument is that in this league it seems that $6M of cap space is worth a mediocre 1st round pick... ballpark anyhow. FW's point is that of you are trading JG and leave yourself no cap room to take salary back, you pretty much have to reduce your expected return by the aforementioned 1st round pick. That is a good argument against signing Hall first. Of course, if you sign Hall first, "one in the hand... etc", Hall is off the market, so there is a good argument for that too.
Personally, I am leaning to FW's view that I would want to have maximum flexibility when making my trades to ensure top return, but I can definitely see both sides on this one...