I was talking about this with my friend, and talking through the logic made me sad. This is kind of a funny situation, because I started playing Rock Band last gen on the PS3. But it eventually became clear that the PS3 was getting a sub-par experience vs. the 360, so I spent a lot of money re-buying instruments and songs on that system, and gave my PS3 games and instruments to my nephew (which means they're destroyed by now). My friend also made that migration.
Anyway, here's the progression of our thoughts as we talked it through.
1. I'd like my DLC to work...but wait, that DLC is on the 360, and I'd probably prefer to play RB4 on the PS4. Well, maybe this will be my go-to XB1 game.
2. I'd like my instruments to work...but they're 360 controllers, and that's not likely to happen; Microsoft would have to allow it. The PS3 instruments might work on PS4, since they're USB...but I don't have any.
3. Okay, scratch all that. New game, new instruments. I'll just play the old stuff on the 360 if I want to play it. I still do, after all.
4. Wait a moment...new instruments. New Rock Band instruments. I'm using Guitar Hero guitars because the Rock Band instruments are all terrible. They never made a single decent one. I went through so many, and they are ALL. COMPLETE. SHIT. The Guitar Hero World Tour and GH5 ones I have now are great. I'd almost forgotten all the non-functional, short-lived or laggy controllers I went through over the years. It's going to be a sad, sad day when my guitar controllers finally stop working. They've been champs, though.
5. So...I think I'd rather just have more songs on the 360, thanks. I've spent so, so much money on Rock Band over the years, and it finally works perfectly. Just give me the handful of songs that were, unfortunately, exclusive to the awful Guitar Hero franchise: Guns N Roses (Appetite for Destruction), Hotel California and a few other Eagles songs, Van Halen. That'll do.