That's a tough question to answer since it's a competitor's product, but I'll try to do my best to give some objective differences from my own perspective.
The Elekit in a way is more like what we used to sell back in the day. The kit is offered with some budget iron with lots of hints dropped that you should really go for the Lundahl upgrade if at all possible. We did this with our SEX monoblocks and Paramour monoblocks and we worked with Magnequest to create better iron for those pieces. There are some missing dimensions to the specs on those stock Elekit amps that I would imagine are rather drastically improved with the Lundahl bits.
The schematic for the 8600 is floating around and it has a vastly more complicated power supply than the Kaiju and active biasing circuitry, but the 300B driver is rather simple and multiple stages. I would expect less 120Hz noise making it from the power supply to the speaker terminals. The DC filament supply on this amp is a bridge rectifier and a cap.
The Kaiju has an incredibly simple power supply, very simple 300B biasing, but a more complicated driver stage, which is just a single triode driving the 300B. The increase in 120Hz noise that you might expect to see at the speaker terminals will be somewhat mitigated by the parallel feed output stage. The (available) DC filament supply is regulated and keeps Jac at EML happy.
I believe the 8600 with the Lundahl transformers will make more than 8W (maybe 11?), but it also runs pretty high plate voltage. R114/214 on the 8600 are twice the allowable value on the Western Electric published datasheet, and I have experienced this causing problems with Russian 300Bs in a different amplifier in the past, but YMMV and this is something I have run into in many 300B amps. With the 12AX7 input stage, the schematic shows either a 50K or 100K volume pot, and I would strongly advise against running a 100K pot into a 12AX7.
The Kaiju will make less power and requires a bias adjustment to the driver stage, so I would say it's less user friendly. I think the big advantage to the Kaiju comes if you happen to blow up one of the PC boards, as everything is modular and we can just send out replacement parts pretty easily. I've seen something like this happen from the wrong 9 pin tube being put into the amp, and a repair like that can be done pretty easily by the builder.
