Having owned both (the Martin Thinline Gold + is a rebadged Fishman Acoustic Matrix II with the narrow piezo strip, which I have had in a few Gibsons and Martins, and I have the K&K in a couple of my current guitars), whilst the K&K has strong output compared to most passive pickups, its a fair way short of the output of the Martin/Fishman with onboard preamp.
The K&K also requires a lot more drastic EQ work to make it sing than the Fishman/Martin and has a much lower threshold of gain before feedback, so switching between guitars with the two different pickups would be less than straightforward.
I would suggest going with K&K in both guitars or looking at the Fishman Matrix Infinity which is the modern iteration of your Thinline Gold. Both are excellent pickups, albeit quite different from one another.
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'67 Gibson J45 (K&K)
81 Eko Ranger IV (weird factory Electra pickup)
'95 Gibson Dove (MagMic)
97 Martin D18GE (Sunrise)
01 Takamine EAN46C (Palathetic and CT4B)
'02 Takamine EAN20C (Palathetic and CT4BII)
'15 Gibson SJ200 Standard (Sunrise)
19 Vintage Paul Brett Viator VC Classical
20 Sigma CF-100 copy (Sunrise)
Capos by G7th, amplification by AER.
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