First, it is a bit premature to say it isn't the same as the BB case. We've no idea what they are adding right now aside from a new mechanic (Guard Cancel Overdrive), a new character (Nu), a new story/mode (the remix heart thing), and completely new balance. I could be missing something (and probably am because I really don't follow it all that closely). In any case, we don't know.
That said, it is unimportant. While I do get crabby as fuck at having to buy the same arcsys game twice, whatever. My point is that SG is effectively a new game too and yet is still treated as anathema based on its vanilla showing.
More characters.
A story for each character.
New stage for each character.
New music for each character (is this one right?).
Better balance.
New mechanics (dizzy).
Lobbies (remember, that didn't come until PC launch).
Better training mode.
Voice-packs (eventually)
The major difference is that you didn't have to buy it twice... and if you had, maybe SG would be given better treatment.
Like I said, I enjoy P4U quite a bit, but it is effectively getting a second chance. When we watch it at EVO, it is going to look and feel similar to its P4A incarnation with a few more characters and better balance. We're not talking a new game. We're talking Ultra Persona 4.
Of course, I'm not saying P4U needs to be treated worse for all of this (I'd be thrilled if BB, GG, P4U, UNIEL made main stage)... but SG needs to be treated a little bit better than 2014 EVO. Hopefully, we get some dope treatment for last years fuck up, but only time will tell.