Mattermost
According to specs.
Hardware requirements for team deployments
Most small to medium Mattermost team deployments can be supported on a single server with the following specifications based on registered users:
1 - 1,000 users - 1 vCPU/cores, 2 GB RAM
1,000 - 2,000 users - 2 vCPUs/cores, 4 GB RAM
Campfire
According to their promo page:
| Concurrent Users | RAM | CPU |
|---|---|---|
| 250 | 2GB | 1CPU |
| 1,000 | 8GB | 4CPU |
| 5,000 | 32GB | 16CPU |
| 10,000 | 64GB | 32CPU |
So, I can make a conclusion that Mattermost is much more efficient. While being packed with much more features.
However, since I have an old Intel Atom server with 2 GB of ram and 2 CPUs, I can try both, considering both of my organisations are less than 250 users.