I know that the main idea against London System is to play Qb6 to attack the b2 pawn which is unprotected due to the early Bf4 white plays (generally) at move 2. Normally white reacts to the threat with Qb3, covering the attack of the queen on b2:
[Title "London System setup"] [fen "rnb1kb1r/pp2pppp/1q3n2/2pp4/3P1B2/1QP1P3/PP3PPP/RN2KBNR b - - 0 1"]
I also know that taking the queen here is considered not a great idea for black... This is due to the great activity white's rook on a1 would gain after axb3.
In this position instead (reached from the Queen's gambit declined opening), Qxb6 is considered the best move for white by Stockfish...:
[Title "QGD setup"] [fen "rn2r1k1/pp2bppp/1qp2n2/3p3b/3P1B2/1QNBP2P/PP2NPP1/R4RK1 b - - 0 1"]
Someone could please explain me what is the rationale behind this? Why in a case taking the queen (and so allowing great activity of the rook on the open file) works and in the other case doesn't? And, in the case it doesn't work, what should be the right plan to follow?