I would guess it ultimately depends on how much you’re interested in modding, fine-tuning settings and calibrating the printer to perfection. If you only want to load a model and hit “print” then I guess you wouldn’t notice much difference from just a mainboard swap. But since you’re about to install Klipper, you probably have some interest in spending time optimising your printer :)

I’ve upgraded a lot of things on my printer, but not got to the mainboard yet so it still has TMC2208 driver hardwired in stealthchop mode. I never got much out of pressure advance while the printer was still in bowden mode, even with a BMG extruder which is much stronger than the stock extruder… Can’t say if it would’ve turned out better with TMC2209 drivers, but after modding the printer to direct drive I have no issue with pressure advance. Now the main reason I want to upgrade mainboard some day is that I would like a 5:th driver so I can do proper dual Z.

And a general Klipper recommendation, in case this is the first time you try it out; Resonance compensation is like magic, and it works perfectly fine calibrating it manually (i.e. without buying an ADXL). After calibration, I could increase acceleration to 4000 without visible ringing. For reference the default acceleration for Ender-sized bed slingers is usually 500, which IIRC can be upped to about 700-900 before you start to get ringing… Not only does high acceleration make it print faster, but at least for me it had a bigger impact on corner quality than pressure advance ever had.