Copy Songs from One iPhone to Another iPhone

The only way to get itunes content on your new phone is by syncing or manually managing songs & videos through itunes. You can sync as many devices as you want to the same itunes account, there is no limit. Unless you created playlists(smart idea), then you have no choice but to go through itunes & select the content you want on your new phone or if you have checked Sync only checked songs & videos. From your post Im guessing you have manually manage music & videos checked, correct? If so, you should be aware of the fact that there is absolutely no advantage to doing so with an iphone. Unlike disk wheel ipods, the iphone does not support disk mode. Thus, youre better off creating playlists & syncing those to your phone.
MacBook Pro

I was afraid that would be the case. I will probably just go through all my songs in iTunes and make a playlist of my favorite ones (maybe just make a playlist based on rankings) so then if this ever happens again, all I have to do is copy the playlist over and not reselect all the songs. Will that work?
Mac OS X (10.5.7)

Yes.
I manage two iphones, an ipod touch and two ipods using playlists. It really simplifies things.
powerbook g4, iMac 20 Mac OS X (10.6.4) Tie Capsule, iphone 3gs, iphone 4, ipod, ipod touch

Yes. Like I said, IMO, the easiest way to manage music & videos on the iphone is to create playlists. You can add/subtract from the playlists whenever you want. No benefit whatsoever to manually managing music & videos since the phone does not support disk mode, & you have to sync everything else.
MacBook Pro

Ok, so I should just set up my iPhone to sync with only certain playlists and not the whole library? Im sure there is an option for that in iTunes but I dont have my Macbook with me right now.
Mac OS X (10.5.7)