The WE Hi-Capa is their longest running design/variant. You'd think they'd have all the kinks worked out, however the WE Hi-Capa does employ a heavier slide, so gas consumption does increase, but all the internals of a WE Hi-Capa are 100% compatible with TM aftermarket parts, the KJW is less compatible, but is still able to use most parts.
I think if you're planning to run a Hi-Capa on CO2 you should pick a KJW version. They already come upgraded with the red loading nozzle designed for use with CO2. WE Hi-Capas were designed to run on propane and if you choose to run it on CO2 the nozzle will break from the added pressure CO2 delivers.