Fabric might be easier to implement, but it pretty much restricts the usage of it to vanilla Minecraft. With Forge it would be compatible with most mods and modpacks.

I have heard that there will not be a forge version of Replaymod, but there is something called PatchworkMC, which is in the early stages of development. It attempts to run forge mods on fabric in the future.

Here is an explanation of why it is not on forge 1.14.4