metro vs qmetro??

Metro is in the high priority scheduler thread which doesn't allow non-timing operations to interrupt its process(es), while qmetro is deferred to a lower priority thread where it will perform its task as the entire processing allows(can make things a bit less CPU intensive).

Use metro when you need timing to be critical(music, rhythm, triggers with specific and reliable frequency, etc.).

Use qmetro when you simply need something triggered frequently, but the timing does not need to be tight(example, if you trigger video frames or matrices at a certain rate in Jitter, the visual motion in continuously triggered frames is not as dependent on a strict timing... video at around 30fps might not look much different if each trigger might be off by a little).