As far as I know all 1/12 use same size rear 碳軸. Had use serpent, 5.1, CRC diff in all these car. But one thing is you may need to use the complete rear diff with the hubs. And need to use washers to space it properly as the motor pod offset are slightly different between different brands.
If only 2 options, I suggest serpent
If you really want crc, I suggest you to buy additional car for spare parts.
Not sure you can buy 2 crc at the same time
Remember: Don't rely on single source.
I bought 2 GenXL. One for spare part, because if you buy all the spares like the screws, bearings, arm, shaft...etc. It is easily more expensive than buying another car.