I think Pederson is the better sign if the team wants a Lefty DH that only plays the field sparingly.
VS RHP
C - Jansen/Kirk
1b - Vlad
2b - Biggio
SS - Bo
3B - IKF
OF - Varsho, KK, Springer
DH - Pederson
Pinch hit for whatever lefty you need to late in the game with the backup catcher, Schneider, Clement or Espinal and if you're not pinch hitting for one of KK or Varsho, you get slightly better D at 2b late in the game. Pinch hitting for the OF and your OF takes a hit defensively for sure unless they manage to keep around a guy like Lukes, which I doubt.
VS LHP
C - Jansen/Kirk
1b - Vlad
2b - Clement/Espinal
SS - Bo
3B - IKF
OF - Schneider, Varsho/KK, Springer
DH - the catcher who isnt starting or whomever of Clement/Espinal isnt at 2b.
Gives max flexibility to PH later in the game for either the DH, or Clement/Espinal with one of Biggio, Varsho/KK, Pederson and get better defensively for the late innings.
Can throw Pederson in LF in Fenway or some other stadiums where the D doesnt matter so much anyway and give whatever OF that needs it a day off.