First, what features are the same across all five devices? They share Samsung's four-inch Super AMOLED display. Based on my experience, Samsung's Super AMOLED displays are among the brightest and sharpest around. They simply look stunning.
All Galaxy S variants also include Samsung's 1GHz A8 Cortex Hummingbird processor, Android 2.1 operating system with TouchWiz 3.0, 5 megapixel camera with 720p HD video capture, 802.11b/g/n Wi-Fi, and Swype software for the on-screen soft QWERTY keyboard.
All five will be among the first to ship with Bluetooth 3.0, which offers some unique features, such as high-speed data transfers between devices.
AT&T's Captivate has tri-band HSPA 3G on board and includes 16GB of internal storage. According to AT&T, it lacks a lot of the custom software that Samsung has developed for the Galaxy S. It also doesn't appear to have a front-facing camera.
Sprint's Epic 4G comes with WiMax, making it the second 4G phone for the Sprint network. Its other defining characteristic is that it is the only Galaxy S device with a slide-out QWERTY keyboard for messaging. Sprint showed me the Epic 4G. Despite the addition of the keyboard, it is amazingly thin and light. The Super AMOLED technology goes a long way to help the Galaxy S family remain slim. It will be boxed with a 16GB microSD card, but doesn't appear to have the built-in storage that the Captivate from AT&T does. The Epic 4G will, however, come with a user-facing VGA camera and Qik's video sharing software. It will include Samsung's Social Hub and Media Hub to help users aggregate social networking and content.
T-Mobile's Vibrant has HSPA 3G inside and will be boxed with a 2GB microSD card. It lacks a user-facing camera, physical keyboard, and Samsung's Social Hub and Media Hub software. It will, however, be preloaded with the movie Avatar and a free month of Go-Go in-flight Wi-Fi service.
U.S. Cellular's version of the Galaxy S will simply be called the Galaxy S. That's OK. It comes with fewer features than its competitors do, such as no user-facing camera and no keyboard. It won't be boxed with a microSD card (according to what we know), which is actually kind of lame. This is the weakest version of the bunch when it comes to extra and/or defining features.
Verizon Wireless' Fascinate's biggest leg up on the competition is that it will come with mobile hotspot creator software on board, allowing the Fascinate to act as a Wi-Fi hotspot for up to five devices. It won't have a user-facing camera or a keyboard, but it is packed with Verizon applications. Some of the apps include V CAST Video, Rhapsody, VZ Navigator, as well as Samsung's Social Hub, Media Hub and support for DLNA content. It will be boxed with a 16GB microSD card, and supports cards up to 32GB.