VII started the PS1 era. XV is starting the PS4 era.
VII was groundbreaking in terms of graphics. Being the first 3D in its time is pretty amazing. XV is pretty incredible in the graphics department. These characters and settings look the most realistic out of any game i've seen.
VII has some of the most popular and iconic characters in the series. Villain is probably the most known FF villain. XV, we don't know much about them yet. Lots of people fangirling over them already, but their fangirling may end soon when we find out what Noctis and Co are actually like. Villain isnt known yet, so thats another iffy thing.
VII gameplay is kind of clunky (imo). Wasn't really a major step for Square in that department. XV gameplay is a whole new change. Based on what we know, it seems like it could be revolutionary; not for the ARPG genre, but at least for SE.
VII story, you will receive mixed opinions on. For me, I feel like it boils down to "just chase the bad guy while being thrown into random subplots that arent fulfilling." Still has some interesting aspects, but it isnt my favorite. XV, idk.
There's potential for it. They just need to do it right.
For me, XV will probably out do VII, because I don't hold VII in high regard.
It's not nostalgia. We just want a NEW FF game in the style of the good FF games like 4 - 10. 12 was ok, 13 wad garbage. Three garbage games and an MMO as their last four. 2001 since they last made a masterpiece FF, suggests to me that FF is dead. 13 murdered the series: 12 was passable.