Even without the sale the Mighty is significantly more expensive, and while it has some advantages (like I hear you can’t really burn your material in the Mighty even with bad technique and it’s from a big company if you like that), I think the Firewood 7 has a lot more going for it.
Stock issues are partly due to the sale, and I think also due to it gaining some well earned attention here, on Reddit, etc. I suspect that if you miss it though, once the sale is over and he has time to produce some more it shouldn’t be too hard to get ahold of one.
Personally I would go for the FW7 right now even if the Mighty was cheaper. The replaceable battery, size, materials, cleaning requirements, and from what I’ve heard even performance are all things that make the FW7 much more appealing to me. I also prefer buying from small makers like Marc as long as they support their products well, and from what I’ve seen he does.