Not GP, but ~15 years ago tmux offered a lot better screen splitting options and a few other features that screen did not. That's why I switched. I did continue to use screen at work because it was ubiquitous, but then sysads started installing tmux as well and that was no longer an issue.
screen has improved a lot since then, but it lagged behind substantially for years.
Also a benefit was actually being able to understand and edit my conf file without having to RTFM everything. Certainly not a reason to switch by itself, but a nice plus.
screen has improved a lot since then, but it lagged behind substantially for years.