Personally if I had a choice of just one I’d pick the threads over fibers if the threads facility is really high performance like the one in Java.
Today you have quad cores even on a cheap phone and 16 on a desktop and 60+ on a server and OS and some runtimes make it pretty easy to get large speed ups on many tasks even when subtasks are closely coordinated. (One thing I like about Java is that it has low-level thread primitives that really scale as opposed to many systems have have a small set of low-level primitives that in theory let you do everything but not scale.)
Contrast that to fibers and similar things (JS and Python async) that do a great job of keeping a CPU busy when you are waiting for network activity but are limited to one CPU.
Today you have quad cores even on a cheap phone and 16 on a desktop and 60+ on a server and OS and some runtimes make it pretty easy to get large speed ups on many tasks even when subtasks are closely coordinated. (One thing I like about Java is that it has low-level thread primitives that really scale as opposed to many systems have have a small set of low-level primitives that in theory let you do everything but not scale.)
Contrast that to fibers and similar things (JS and Python async) that do a great job of keeping a CPU busy when you are waiting for network activity but are limited to one CPU.