I will say definitely go for the iMac.
I think its about the same price as the iPhone if you consider the monthly bills, so its not a matter of picking a 2000 gift or a 200 gift, they are both the same value in the end.
If you are into video editing and stuff like that, you will definitely benefit from the iMac. Besides, if you learn how to do really good video editing you can turn that into money down the road. You will be developing a great skill that will be very valuable your whole life.
If you get an iPhone, you will get a cool piece of technology, but ultimately it is overpriced and unnecessary for your needs. Get a summer job or mow some neighbors lawns and save up for a 2nd hand iPod Touch, if you need extra storage so badly. Or just pick and choose which items from your library you want to listen to at any given time.
Its nice to have your WHOLE library with you, sure, but no human has a need for more than maybe 512MB or 1GB of music at any given time. Every week just decide which few albums you will be listening to and synch those. Then rotate bands in and out. 4GB is more than enough space.
As for phone, get a prepaid phone.
They come pretty cheap, I am sure you can snag one for under $60 and get recharge cards for $20 that will last you a while (assuming you only use the phone when necessary and "to have a number" not to talk obsessively on it).
With the phone, you're going to get something that may seem cool now, but does not add any value to your life. With the computer, you are going to be developing a useful skill (and possibly other skills as well), so the benefits are much greater.
You will also spend far more time on the computer than listening to your mp3 player I am sure.
