What do you mean unrelated stories? Every single SMT/Persona game has a separate story. What makes the game unique, story-wise? We don't know yet. What makes the game unique gameplay-wise? From the looks of it this game has plucked gameplay/setting elements from many different MegaTen games. The destroyed city is from SMT. The "two different worlds" and changing main characters from Persona. Social sim (if there is any) from Persona. We have no idea how this game's multiple endings will likely work (if there are multiple endings.) We have no idea how dungeon crawling will work, yet (which will tell us whether or not it has plucked it from SMT or Persona.) Like DDS, Devil Survivor, etc this game seems to have some MegaTen elements, while discarding others, and at the same time adding additional things (Fire-Emblem's class system, ATB battle system, larger open/semi-open world, etc.) 

This is obviously not a spin-off (in the story sense), precisely because the story probably won't connect to any Fire-Emblem or SMT game. Fire-Emblem characters will likely have the same role that real-world mythology has in mainline SMT games. 

If this not spin off why there are alot of FE and SMT old character in this games and also "because the story probably won't connect to any Fire-Emblem or SMT game" (that's whta people call spin off ) ? and then the gameplay is more of SMT or Persona games rather then a new creation for cross game mechanic, it doesnt add new thing and look like another Persona and SMT while loosing many from FE. while Persona 5 it Persona series people already love. and if you say SMT and Persona has separate story it's wrong they have seperate stories but it still on the same universe except Digital Devil saga which is an easter egg for dante.

Which SMT character from another game is in here? As for Fire-Emblem, it is obvious that the characters are part of some sort of lore in the same way SMT takes from real world lore (demons in SMT games are gods/goddesses and monsters from real-world lore.)  Why does the game play have to be vastly different for the game to be good? Almost every modern SMT game and Persona uses the press-turn system, it doesn't look like we have press-turn system here, and we have fire-emblem classes. Is that not different enough? Your main argument was, "this is just a cross-over game therefore it isn't as good", and then you turn 180 degrees and say it isn't cross-over enough because there isn't enough fire-emblem. You can't choose both arguments. The stories in the Persona/SMT games are still separate, even if there are some slight connections hinted in the neutral endings of SMT games. As for the universe, we don't know what universe this game takes place in. It could be one of the many multiverses that YHVH is creating. 

Since they are both spinoffs technically the correct term is Established Franchise vs New Comer. Especially since its not straight up FE or SMT but a selection of qualitys of both, you can't count on either fanbase, rather its mainly those who are hyped about the game itself. In contrast, this is P5 which even from P3 has been established since PS2, with its own spinoffs being on 360, PS3, and DS. So yah in terms of sales yeah, its gonna be no contest.

If we are talking about what SMTXFE has to offer, we are assured it won't be held back by last gen tech, although Atlus is a master of art style so this might not matter as much. It will probably have an effect on explorability, etc. though.

As for the screens, I don't know if we've seen enough yet. It definetly has ATBS but what does normal combat look like? It says Scene up at the top right, will these areas be only accessible in certain story parts? How can we know if it won't be ffxIII corridor esque? Still many questions.

OK i am surrender , it's getting hard to discuss, because of language barrier on my side,  in my perspective SMT X FE is more like Capcom VS SNK game, it's more of SNK playing mechanic rather then inventing new mechanic for both different games . While people will definitely compared this games with Persona series because it already surpassed SMT mechanic and people already comfortable with it and only small FE loyal fan base will interested with Persona games mechanic. that's why this game will be just another Persona games with FE character. and don't have originality. it will be different if SMT X FE creating new mechanic  not from Persona/SMT nor from FE. That's why most people have a mix feeling with this games. They were expecting this will be totally new genre and mechanic but the reality is it's just another persona/ SMT games with FE character = spin off .

So to summed up with OP, does SMT X FE look better then Persona 5, yes it look better (slightly or not) from graphic stand of point, but gameplay it looks very Persona.

This is reminds me a lot of the whole FFXV vs XCX debate.

Let's be honest, if you love one -- you'll probably love the other.

But we can almost guarantee that the quality of this game will be at least good as there haven't been any bad SMT games (spinoff or otherwise.) Most of the series is reviewed > 80%. The game is obviously more SMT than FE, so I don't think it has the issue that other crossover games have. Nobody was really discussing sales. Persona is the best selling MT series. 

As for explorability and world size, I think Persona 5 and this will probably be only limited by costs. Both the Wii U and PS3 should be capable of open-world/semi-open world gameplay. 

Sure we don't know what it's like, but why would we assume that it will be bad? We know it's not FFXIII corrider-esque because we've seen what looks like an open-world city with shops to buy things in throughout the trailer. 

^ All show some pretty open and interactive environments. Notice the demonic gate in front of the large building in the second picture. That implies backtracking is present in this game as well. 

Have you played a Persona title? Those gates are pretty identical to what you see if every Persona title. I don't think it's any indication of backtracking. Just that it's blocked progression or holds a boss behind it.

They're present in both Persona and SMT games. Often though you want to go find items within the dungeon before you enter the gate or you need to unlock the gate (in other words: backtrack.)