An oversimplified explanation is as follows:
Bitmain, an ASIC manufacturer, created a Cryptonight ASIC. This enabled the company to "control" a large portion of the Monero hash rate and consequently mine many coins.
When The Monero Project learned about this, it modified the Cryptonight proof-of-work (PoW) so that any previously-created ASIC's would be rendered generally ineffective.
The PoW change was made effective as of the Spring 2018 Monero non-contentious network upgrade (which requires a hard fork), at block 1546000. The transaction version as of block 1546000 is
v7. The Monero Project is currently developing on top of this (and its users are using it too, of course).
Bitmain didn't actually fork Monero. They simply neglected to update. They are using deprecated code and continuing to mine on an invalid fork. Granted, if enough people treat the fork as valid, then it becomes "valid" in that sense.
As far as I know, there is no development on the deprecated code. Perhaps if there was a legitimate reason to maintain the deprecated code, someone would have at least changed the network ID so that the two forks wouldn't confuse each other's nodes, which has happened. Since the deprecated code does not appear to have maintainers (and since the only apparent reason for that fork to exist is so a company can mine it more efficiently than anyone else), it may make sense to treat that fork as invalid.