In its current state, Replay's not meant to replace either of those products as it's not meant for 24/7 user session monitoring. Instead, it's something that we see teams reaching for in their debugging workflow.
The usual bug workflow is for one person to file an issue with steps to reproduce, which the engineer will use to reproduce the bug and try to debug the problem. Replay replaces that workflow by allowing a person to record a replay, and send that link (which is immediately debuggable) to the engineer.
Replay employee here. Personally, I'm excited to see maintainers slowly phase out "Please include steps to reproduce" in issues, and replace it with "Can you make a replay"?
Partly because I want to see the product do well, but also selfishly as an engineer, because we've been dogfooding replay and it's made squashing bugs 10x easier. Having somebody attach a replay to an issue makes that issue immediately better than an expertly-written one, which as an engineer, I can start debugging in seconds with minimal back-and-forth.
However, we do need you to sign up for an account to record a replay.