I think
oxygen is the most important thing because it enables you to avoid ice in the first place, and most flights that are done away from frontal weather can be "arranged" to avoid spending much time in IMC.
An N-reg TB20/21 with TKS is not FIKI certified, but a G-reg one is - the opposite of what you might expect. The FAA want a second alternator and some other stuff.
I have no experience of this myself but owners report that TKS is much more effective than rubber boots. But... when the fluid runs out, it's gone. So it's a matter of strategy.
It seems readily apparent that when one looks at nonpressurised owners of these two systems, the TKS pilots use it to get through layers, and most use oxygen to fly VMC on top (like I do but I have only a TKS de-iced prop). The booted pilots often seem happy enough to just keep flying in IMC, at low levels, collecting a bit more ice here and there.
I don't think it is a total coincidence that planes over 1999kg, which pay Eurocontrol IFR charges, have mostly boots
If you are flying "VFR" in IMC for hours, then you want boots, not TKS. Of course a part of that is that they tend to be older designs which pre-date TKS.