A buyer's guide to the new iPhone 4S

- If you own an iPhone 4: The new phone will have a faster processor and a sharper, more responsive camera. Otherwise, it's pretty much the same. It will come with improved software, but you'll get that as a free update on your iPhone 4, too.

As an iPhone 4 owner, you should consider the 4S only if you absolutely must have the latest and greatest, or if your old phone is broken. Since the 4S is less than two years old, your carrier will probably make you pay more than the $199 base price if you upgrade, because you haven't "paid off" the subsidized price of your old phone yet.

There's speculation that the more significant iPhone 5 upgrade may be less than a year away, and it could add important new features that are worth waiting for.

- If you own an older iPhone: Apple's new software, iOS 5, will work on the iPhone 3GS, but not the original iPhone or 3G. Take the launch of the iPhone 4S as a good opportunity to upgrade to a faster, more responsive phone, with a sharper screen.

The big cost of owning an iPhone isn't in the purchase price, it's in your monthly service fees. Upgrading your phone every two years is a minor cost compared to paying your monthly bill over the same period. So take advantage of your carrier's phone subsidy and let it treat you to a new iPhone. Because Apple charges about $600 for a phone that costs $199 in the store, it's the phone company that eats most of the upfront cost of the phone.

- If you don't have a smartphone: iPhones are still the kings of the smartphone world, with unsurpassed access to high-quality applications. But they're also expensive. That may not be immediately obvious in the cellphone store, where their $199 price tag (or, in the case of the iPhone 3GS, $0 price tag) looks comparable to many other phones. Carriers require contracts when selling iPhones at that price, and the available plans aren't cheap, in part because you'll need a data plan. In effect, you'll be paying off that expensive phone over two years, through your monthly bill.

If you want a smartphone for less, look at getting a handset that runs Google Inc.'s Android software from a no-contract carrier like Virgin Mobile, MetroPCS or Cricket. You'll be paying $149 and up for the phone, but the monthly cost will be lower. The biggest weakness of Android phones is that there are fewer good third-party applications available for them, but you'll get roughly 80 percent of the functionality of an iPhone for 50 percent of the cost.

- If you're a Sprint subscriber: If you've nursed a longing for an iPhone but haven't yet moved over to AT&T or Verizon, now's your chance ... probably. Sprint hasn't yet said what kind of plans will be available for the iPhone. It's also not clear if Sprint will sell only the 4S or also the cheaper 4.

But it's a fair bet that Sprint will keep its unlimited data plans as a way to lure subscribers from Verizon and AT&T, which cap monthly data usage on smartphones.

It's worth noting that the iPhone won't support "Sprint 4G," which is what Sprint calls Clearwire Corp.'s high-speed data network. Sprint sells a number of other smartphones that can access this network for faster Web browsing, downloads and uploads, for no additional cost.

On the other hand, the Sprint iPhone 4S will most likely be able to roam internationally, a very rare feature on Sprint smartphones.

- If you're an AT&T subscriber: The Big Orange was the first carrier to place caps on the monthly data consumption of its iPhone subscribers, to keep them from overloading its network. The addition of Sprint to the stable of iPhone carriers will probably give you the option to jump ship and get an unlimited data plan, but Sprint hasn't confirmed this yet.

On the other hand, Sprint's data speeds are lower than AT&T's, and it doesn't have the network of Wi-Fi hotspots that AT&T does.

AT&T will still be the only U.S. carrier to sell the iPhone 3GS, a two-year old model. The price will be cut to nothing, from $49. But AT&T still won't sell the phone for use on prepaid plans, which could yield substantial savings.

- If you're a Verizon subscriber: One thing that's missing from the iPhone 4S is the ability to tap into Verizon's latest high-speed data networks, which uses the so-called "LTE" technology. That feature might arrive in the next iPhone, which presumably will be the iPhone 5. There's speculation that we might not have to wait until next summer to see that model.

Apple is usually conservative about adding new wireless technology to its phones, but Verizon has been selling LTE phones for half a year already.

If you travel a lot, it might be worth upgrading from the 4 to the 4S because you'll be able to use it on wireless networks in other countries. But the upgrade will be expensive, because Verizon iPhones are so new. You haven't yet "paid it off" through your monthly service fees.

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