I suggest you buy a local SIM, otherwise roaming charges will kill you. Think $1-2 per minute, or more, depending on your home carrier.
You will have to choose between AT&T or T-Mobile, they are the two (main) GSM providers. Both offer nationwide prepaid plans. Your N9 will work on both networks in 3G mode, your iphone will work on both networks in EDGE mode and on AT&T in 3G mode. In urban areas they're probably similar, but AT&T has significantly better coverage in rural areas. You can check their website for coverage maps. You can also check their website for pricing on prepaid plans (called gophone on at&t and pay as you go on tmobile).
I am more familiar with AT&T plans, since that's what I buy for my parents when they come to visit from Europe: the SIM costs $25 to buy, but it gives you $25 credit to your account so it is essentially free. Some clueless salesperson might want to convince you otherwise, but prepaid SIM card purchases are so rare they don't really know what's going on. You have a choice of plans, $0.10/minute, $0.20/SMS or if you're going to use your phone a lot, $2/day for unlimited voice/SMS access. You can also add a data package to this, which will cost you extra.
One thing you need to know about US cellular market is that you pay for incoming calls and texts at the same rate as for outgoing calls. This is compensated by the fact that rates don't differ if you're calling to a different network, either cellular or landline. Obviously, this does not matter if you decide to go for the $2/day unlimited plan.
T-Mobile's plans are different, I don't think they offer an unlimited option, and they are a bit more expensive, depending on your usage. Also, not sure you can get a data plan with them.