I will agree that Sentai is generally less diverse than PR given the series has a had a much longer run, but they have had female White, Silver and Red rangers many of them being important 6th rangers. If we count Gokaiger then we can say they've had female versions of all colours :)
Games should try to be as inclusive as possible within the frame of their design. Obviously, I haven't played SMTIV, but given the track record in the series I doubt the characters sex is a major factor and I think it would be an excellent option. One thing I really like about SMT games is that despite making their main characters blankslates the MCs have personality and by the end of the game no matter what route you picked (at least in my experience) it makes sense. Giving people that option if it makes it easier for them to invest in the character would make the experience better for a number of people.
I can get not wanting to spend or not having the resources to make two distinct characters (because I imagine they would want to make changes like in P3P) especially for a small company, but in a world that needs more female heroes in general this kind of attitude makes me a little disappointed in Atlus. I don't think this matter too much for this game, but just knowing that is the prevailing mindset bothers me. Still excited as hell for this game, though.