I still get overall more fights on the ps3, and by fights I mean rank. Which makes sense because still so many people have a ps3. Right now I'm waiting in rank on the ps4, and it takes so much longer, but this is my experience. Overall get a ps4 because you want it, and at some point everyone will finally move over to the ps4, but honestly IMO no rush.
If you wait the only thing you'll get is maybe a cheaper pirce in the future depending how long you wait, but if you get it now you'll just be waiting for everyone else to move over. So yea no rush lol.
If you want to play other games aside from Last Round, then 2015 is going to be full of great releases for the PS4. If it's just to play Last Round, there is no need to upgrade at this point in time.