I think ETH would be a good choice. The crypto market is moving forward and ETH and BCH both coins gained a new high, which is very good for the crypto market. Though BCH is a good coin, but ETH is more stable than BCH and ETH will have a new update very soon. So, overall, ETH will be a good choice to buy and hold for a month!
Ethereum is a very profitable long-term investment, but if you start now maybe in 1 month you can get results from it because the market is currently moving well and in 1 month Ethereum may be strong enough to soar.
no movement knows and maybe BCH can also be profitable so investing in both will be better.