With yesterday's announcement of the Motorola Droid X on Verizon, and first shipments of the Apple iPhone 4 on AT&T joining the HTC Evo 4G on Sprint, three of the four big U.S. carriers now have new flagship high-end smartphones.

With yesterday's announcement of the and the on AT&T joining the on Sprint, three of the four big U.S. carriers now have new flagship high-end smartphones.

Some things are constant among these phones. First, all run 1GHz processors: The iPhone runs Apple's A4, the Evo runs the Qualcomm Snapdragon, and the Droid X runs on a TI OMAP processor. All add much better cameras: A 5-megapixel one on the iPhone 4 and 8-megapixel ones on the Droid X and the Evo 4G. And all have bigger or higher-resolution displays and a large selection of applications. But while there are a lot of similarities, what interests me most are the differences.

The iPhone 4, of course, runs Apple's operating environment, now called iOS 4, and AT&T is the only U.S. carrier to offer it. (I do know people who run "jailbroken" versions of the earlier iPhones on T-Mobile, but not many.) The new phone looks great and is very thin, and its 3.5-inch LCD has the highest resolution at 960-by-480, though the screen is physically smaller than that on the other two devices.

Although the hardware looks great, the biggest benefit the iPhone has is still its integrated ecosystem, with hardware, software, and the App Store all working together. There are more applications in total - and more good ones - in the App Store than for other mobile platforms. I love a number of applications, and find its mail program to be better than the defaults on other platforms (particularly now with the addition of a unified inbox.) The iPhone 4 adds a front-facing camera, and FaceTime video calling looks easier to use than other choices, though I want to try it more.

The downsides: It's a closed environment, so Apple controls what goes into the App Store, sometimes with unpredictable results; AT&T's known issues with dropped calls (which as I've said seems to be improving, but it's clearly been a big problem); the iPhone's lack of expandability - neither a removable battery or an additional memory card; and its lack of support for Flash, which means that some Web sites (such as Hulu) don't work.

The other flagship phones all run Google's Android operating environment, which is notably more open: You can choose from multiple handset makers and carriers, and the Android Market is notably less restrictive than Apple's store. There aren't quite as many applications there, but there are tens of thousands, and most of what you want is available.

But a downside here is that these phones tend to be a bit more complex. The user interfaces aren't as consistent, and most of the hardware makers add their own enhancements that may or may not conflict with those offered by the OS and the carrier. You have more control, but you have to work at it more.

The Evo 4G has a large 4.3-inch display, with an 800-by-480 resolution, and as a result, the phone is a lot larger than the iPhone 4. It has a variety of very nice features, including a front-facing camera for video conferencing and the ability to be used as a mobile hot-spot, sharing its Sprint connection with multiple computers over Wi-Fi. It also has an 8-megapixel camera, HDMI out, and a kickstand. But the standout feature is its support for the Wimax network, which is now available in a number of markets and coming to a lot more this fall.

The downsides include size - the flipside of the big screen is that it is notably bigger than the iPhone and feels heavier than the Droid X - and for me, at least, battery life and a number of little bugs I'm waiting to get resolved. Currently, Flash doesn't work, but Andorid 2.2 with support for Flash 10.1 is likely coming.

The Droid X, like the Evo, has a 4.3-inch LCD but with a slightly higher resolution of 854-by-480, and ships with Android 2.1, with the 2.2 update and Flash promised for later this summer. (The device is scheduled to ship July 21). It too has a wireless hotspot feature, removable battery and expandable store.

A couple of things make it different, including its own set of Motorola widgets that go on top of Android and an optional larger battery that makes it a little thicker. But what really stood out to me was Motorola's focus on voice quality, with additional antennas and three microphones for improved noise cancellation. Of all these phones, it is the biggest, slightly longer than the Evo 4G. I haven't been able to use one yet.

These phones will be soon joined by the , originally with the model on AT&T, but rumored to be on other carriers as well. This too runs Android 2.1 initially, but has Samsung's own 1 GHz processor and a 4-inch 800-by-480 "Super AMOLED" display.

One thing I've noticed is how much lighter it feels than the other big Android phones, but we'll have to see what that means for battery life. And I really want to see how this OLED display really stacks up to the others.

Also note that T-Mobile has a number of interesting phones, including the HTC HD2, which runs Windows Mobile 6.5 and a number of Android-based models, although I'm not sure I'd describe these as flagship phones in the same way.

I can't say any one phone is perfect for everyone. Some people will prefer smaller phones, and others will insist on physical keyboards, as on the BlackBerry. Some will balk at the data plans. And everyone will have to choose not just a phone, but also a carrier, which offer different levels of service, coverage, and plans.

Originally posted to Michael Miller's blog, Forward Thinking.