Not taking into account the rend as that makes things a bit more complicated and it's too early for that, the flamethrower will average more damage once there's 10 or more enemies to hit with it. Anything below that the crossbow wins. Taking into account the rend the flamethrowers will probably need like 2-3 more targets to hit to be better.
Also the crosbow does have the awkwardness of only having 3 shots. Having all of em miss in a single round is going to be fairly common due to the low amount of shots, which can feel a tad lame at times.
To be honest, it's a tad dissapointing the flamethrower doesn't just hit everything in range, or function like a dracoth's breath attack where you just hit everything at a certain point. It'd help with dealing with loads of small units who together are still effectivly a horde. Right now it can still get swarmed despite having an anti-horde weapon which feels odd.