By name alone along with Nintendo's typical lousy marketing, Fire Emblem.
Xenoblade if Nintendo somehow managed to do a fairly decent marketing.

FE X SMT.

FE and SMT both have a niche following ever since the Super Nintendo days and it's been only recently for Fire Emblem to break the 1million sales. SMT never sold over the 1 million, so this series crossover might actually help them in terms of sales.

Xenoblade X is constantly being mentioned on Nintendo Directs but i don't know if this game will actually sell that well, just like Bayonetta 2.

Fire Emblem.

Can't tell. Fire Emblem fells like the bigger franchise, but Nintendo is currently pushing XenoChron X much harder.

Xenoblade Chronicles X... Nintendo barely even talks about the  FE X SMT game!

Honestly, it depends on what sort of game SMT x FE turns out to be. If it is in more along the lines of SMT, then I see Xenoblade CX being the bigger seller (at least in the western territories) but if it is more along the lines of FE then I can see it beating out Xenoblade CX.

Presently however, the bigger issue seems to be whether we will ever see this game come to light, it was announced before Atlus was bought out by Sega and early in the Wii U's life, even with the curret situation for the Wii U improving the development of the game may get scraped especially given the lack of any hard details on it.

I think Fire Emblem x Shin Megami Tensei will fail because it is being made by Atlus (not Nintendo) as a normal RPG, not a Strategy RPG.
In Fire Emblem after the first playthrough I skip al cutscenes and dialogue and go straight to the battles, I will not buy this crossover if it is a normal jrpg.

That game is beautiful. Look beyond the character models. Also, this is what the game would look like on the gamecube. Maybe worse.

I used this picture to show the contrast between characters and environments.

they never said anything about it will be a normal rpg or a strategy one. Actually, they said anything but the name

I think we had another thread for this before...but I'll answer again anyway.

Right now, it seems to me that Xenoblade Chronicles X will sell more. Fire Emblem and SMT both have loyal fanbases, but the games themselves are very different, both in setting and gameplay. Unless they find a way to fuse the gameplay styles, it seems to me like one fanbase will be expected to cave to another style or setting. I just can't see fans of both franchises rushing to play something drastically different than what they're used to.

X on the other hand, seems to be following the same vein as its predecessor, a game that was received extremely well both among critics and fans, and is receiving a lot more hype at the moment.

This could change...obviously, but this is just how I see things right now.