A good rule of thumb is that newer devices will perform better than older devices. So if you can test on a 3rd gen device (iPhone 3GS or iPod Touch 3rd gen), then you can be pretty confident that the 4th gen devices (iPhone 4 or iPod Touch 4th gen) will perform at an equal or better framerate.
The simulator is really horrible, as everyone here will agree with. It uses different libraries than a real device, and some of them really don't work well (especially audio). Before you even consider performance, you need to get a real device to test on. I know, the last thing most new developers want to hear is that they need to spend more money first, but it's worth it in the long run.
For example: my current game runs around 35 FPS on the simulator on my MacBook Air. On a real iPod Touch 3rd gen, it is a steady 60 FPS.
(Of course, you will also need to pay Apple their developer license fee to test on a real device, so that's something to remember, as well.)