Both are huge cases. The ATCS 840 is a bit bigger IIRC, but the FT02 isn't small by any means. "Mid-tower" only refers to the height on that thing, as it is extremely deep. Essentially, if you took any normal full tower case and turned it so that the front is on the ground, you'd have the rough size of the FT02.
I can't speak for the ATCS 840 from experience, but I do own an FT02. I actually replaced it with an FT01 due to the size and weight. The case is absolutely unparalleled when it comes to cooling and silence, but it is mostly made of steel (heavy) and it has a few small quirks that pissed me off, such as the inability to mount long optical drives (such as my Blu-ray drive) in the top 5.25" slot - shorter drives will work fine, though. On the other hand, the ATCS 840 is completely aluminum, has a removable motherboard tray, and has a better layout for liquid cooling. It also isn't ridiculously deep, meaning that you should be able to fit it where you'd normally expect a large case to fit.
The final kicker for me was the aesthetics of the FT02. It's not a bad looking case, but the unusual deepness just makes it look like something from a space ship IMO. I replaced it with it's predecessor, the FT01, which has worse cooling capacity and no cable management, but better looks.
Overall, the FT02 is great, but very, VERY different.