BirdNet has been around for several years now, while Merlin just got sound-id recently. Merlin has coverage for about 450 birds of North America, while BirdNet can id around 1000 birds of N Am. and Europe.

With BirdNet you make a recording, highlight the interesting section of the sonogram, and upload that section to the BirdNet servers. With Merlin you start recording and the software ids birds in real time, popping up species as it goes.

My assumption is that, because it runs locally on the device, Merlin is going to be less accurate than whatever BirdNet is able to do on its beefy servers. But it is has the advantage of working without a data connection. Merlin can also id from photos and descriptions.

So the one isn't a replacement for the other. It's great to have options.