kreiff and higher resistance seems to generally equate with higher output
Often this is the case because it means more turns of wire in the coil. It is an indirect relationship, hence often a metric used to express pickup output level. It is very easy to measure with inexpensive tools and minimal skill (compare to the average guitar player measuring output voltage with an oscilloscope).
In principle an op amp buffer will be cleaner and likely lower noise as @giuliomoro advised. Some people like the coloration they get from a JFET buffer, but to my ears it's too subtle to notice.
If you are going with one of the kits you posted, and you intend to use Bela 5V supply, I would recommend the JFET because it is the only one of the two that will work at 5V. You may still need to mess with biasing to optimize the headroom (output at source should be biased to an even 2.5V).
If using an extra 9V pedal power supply with your end system, then the op amp kit will be ok. It will be no good if you intend to use the Bela 5V supply.
If you forget about kits and are up for hacking SMT packages together with flying leads on perfboard then you can find a lot of really great op amps that operate down to 3.3V. Using SMT increases the options available for op amp buffer circuits.