Here are the specs for each phone.
iPhone 4
3.5" Touch-screen LCD IPS display, 960 x 640 resolution
5 Megapixel rear camera, VGA front facing camera
720p HD video recording
A4 Processor
Battery life: 7 hours of 3G talk time
3G and 802.11n Wi-Fi networking
Cellular Signals: Quadband GSM, pentaband HSPA
Bluetooth, GPS, Digital compass, MicroSIM
Proximity sensor, Light sensor, Accelerometer, Gyroscope
Up to 32GB of storage
iPhone 4
3.5" Touch-screen LCD IPS display, 960 x 640 resolution
5 Megapixel rear camera, VGA front facing camera
720p HD video recording
A4 Processor
Battery life: 7 hours of 3G talk time
3G and 802.11n Wi-Fi networking
Cellular Signals: Quadband GSM, pentaband HSPA
Bluetooth, GPS, Digital compass, MicroSIM
Proximity sensor, Light sensor, Accelerometer, Gyroscope
Up to 32GB of storage
Evo 4G
4.3" Touch-screen TFT LCD display, 800 x 480 resolution
8 Megapixel rear camera, 1.3 Megapixel front facing camera
720p HD video recording
1GHz Snapdragon processor
Battery life: 6 hours talk time
4G, WiMAX, and 802.11b/g Wi-Fi networking
Cellular Signals: CDMA, EV-DO Rev. A, WiMAX
Bluetooth, GPS, Micro SD support, Digital Compass
HDMI output
Proximity sensor, Accelerometer
8 GB included (up to 32 GB via Micro SD)
Built-in Sprint Overdrive Mobile Hotspot
4.3" Touch-screen TFT LCD display, 800 x 480 resolution
8 Megapixel rear camera, 1.3 Megapixel front facing camera
720p HD video recording
1GHz Snapdragon processor
Battery life: 6 hours talk time
4G, WiMAX, and 802.11b/g Wi-Fi networking
Cellular Signals: CDMA, EV-DO Rev. A, WiMAX
Bluetooth, GPS, Micro SD support, Digital Compass
HDMI output
Proximity sensor, Accelerometer
8 GB included (up to 32 GB via Micro SD)
Built-in Sprint Overdrive Mobile Hotspot
I think it is really going to come down to which phone company you want to be tied down to for the next two years. I personally hate AT&T, so it will be a no brainer for me to go with Sprint and the Evo.
Hopefully, someday we will be able to choose the phone we want, rather than the phone carrier we don't.