If Dancing Mad were shorter, or it was just certain parts of Dancing Mad that we are voting on, then I'd probably pick it. But there's almost too many themes that don't sound related for it to work as anything other then a medley for me. It sounds more like a collection of different pieces then anything else.
J-E-N-O-V-A's biggest problem is that the OG version is inferior to the ACC version. I listened to the OG version of J-E-N-O-V-A and was prepared to pick Dancing Mad almost by default, but then Dancing Mad just kept going on, and on and the main theme of it doesn't come up until minutes into the piece. J-E-N-O-V-A is at least short and it's got a great intro theme going thoughout the whole piece that has this alien feel to it, especially when compared to the rest of the FFVII soundtrack. It just doesn't sound like belongs with the rest of the FFVII world like the rest of the OST does. Which is perfect for boss fights against an alien.
I kinda feel like the songs are almost too different to compare to each other on equal footing. One is a shortish boss theme for a very specific type of boss fight. And it works very well in that role. The other is a final boss theme that's split up into four different themes for all of it's different forms. It's almost like if Premonition, The Legendary Beast, Maybe I'm a Lion, and The Extreme (the final boss themes of FFVIII) were all presented as one long song.
Listening to Dancing Mad as it is feels like a chore, while listing to the individual themes in it separately wouldn't. Listening to J-E-N-O-V-A never does. For this vote, I'm going with J-E-N-O-V-A, but if Dancing Mad had been split up into it's separate themes as their own pieces of music, I'd likely vote for one of them instead.