There is a similar discussion going on here:
http://androidforums.com/htc-evo-4g/143351-why-buy-over-vibrant-2.html
If you don't want the visit that thread... here is what makes Galaxy S Phones the Gold Standard for Android Phones. Some of the info below is about the vibrant, which won't apply to your Epic 4G. Copied and pasted this from the Vibrant vs Evo discussion.
Lets stick to the facts please. Simply put, the evo is the slowest 1ghz super android phone released, failing miserably in almost every benchmark. Galaxy S phones have the fastest CPU and GPU of any Android device. T-mobile customers have been enjoying 5-7 mbps download speeds on HSPA7.2 for a long time now even on old devices, and now that T-mobile is going to roll out HSPA+ devices, expect 12-21MBPS, and lets not forget, HSPA+ is expected to hit the 41mbps mark. So while we enjoy 5-7 mbps using our old devices, sprint is barely rolling out 4g, which is still slower than t-mobiles HSPA+ network, which again will get speeds of 12-21mbps starting with septembers HSPA+ device.
So lets clear up some facts. Samsung Galaxy S has the fastest 1ghz CPU and GPU. The Snapdragon used in the EVO has been branded the slowest 1ghz chip.
Evo has poor graphics performance. Every online review has shown that iPhones 5 megapixel and Galaxy S 5 megapixel trump Evo's poor performing 8 megapixel. Camcorder in the Captivate is getting wide acclaim. No Flash on Vibrant? No worries, the cam is built with Night Mode Technology, no flash required.
Galaxy S phones are extremely thin and light weight, Evo Feels Bulky.
More Facts. Galaxy S phones use gorgeus Super Amoled screens that are true wide screen and peform stellar under the sun. The image is outstanding. Evo is not true wide screen, the display is not even slightly close to the gorgeus image available on Vibrant.
Memory:
Evo has 1gb ROM, 512 mb RAM
Vibrants memory....drum roll please. 16 gb internal memory with 2 GB dedicated to aplications! Holly Cow! Comes with external 2gb card loaded with Avatar, upgradable to 32.
 
Who has the best Customer Service in the industry. T-mobile. News Flash, T-mobile just won for the 6th straight yr in a row, JD Power and Associates 2010 Award once again for Best customer service. Who has been coming in last place every year for the last decade... aww its sprint again.
http://www.t-mobile.com/Company/CompanyInfo.aspx?tp=Abt_Tab_Awards
 
 
Some Videos to stop this nonsense.
Cnet network data speeds: Sprint last place, T-mobile comes out on top.
http://www.cnet.com/8301-17918_1-20010338-85.html
 
Article: More proof that T-mobiles HSPA+ is Faster than Sprints 4G and available in more cities than 4G.
http://www.thetelecomblog.com/2010/06/05/t-mobile%E2%80%99s-3g-network-faster-than-sprint%E2%80%99s-4g-network/
END OF DEBATE, HSPA+ is FASTER THAN 4G.
Fastest GPU: Samsung Galaxy S phones
http://www.youtube.com/watch?v=cl4p5JI0-gQ&feature=player_embedded
Evo Last place in more benchmarks:
http://androidandme.com/2010/07/news/galaxy-s-lineup-leads-the-pack-in-android-gpu-benchmarks/
Evo poor results.
http://www.youtube.com/watch?v=ilx_cpGZczM&feature=player_embedded
Galaxy S phones Blowing away all phones including DroidX, Samsung Galaxy S phones officially have the fastest GPU and CPU:
http://www.youtube.com/user/androidandme#p/u/1/zgoQHxy-0mM
 
So the evidence and facts are clear. The Samsung Galaxy S phones including the Vibrant outperform the Evo and Sprint in every aspect from camera performance, cpu, screan, gpu, and yes T-mobiles HSPA+ is considered faster than 4g. Lets not forget about 6yrs in a row JD Power and Associates Excellence In Customer Service award to T-mobile.
While Evo has a kickstand that constantly tilts over and falls flat on it's face, Vibrant comes with something functional. A full copy of the highest cash cow movie of all time, Avatar! Avatar takes full advantage of the Super Amoled screen, try playing Avatar on the EVO, and well the experience will be sad.
I see only 1 advantage of the Evo, it has a FFC. Thats it, 1 single advantage. Now lets get real, raise your hand if any of you had a successful face to face video voice call using your FFC. Oh wait, why isn't anyone raising their hands.
Both phones have had issues at release. I wont remind Evo owners of all the issues. I will point out however the GPS has had issues on Galaxy S phones. Not to worry, Samsung has officially announced an update fix will be released soon.
Return the Evo and wait for the Epic 4G.
Last but not least, How could I forget this. Gorilla glass!!!!!!
It is no secret Evo screens shatter, even a gentle 1 foot drop shatters the Evo Screen. Some evo owners have even reported what appears to be a spontaneus screen shatter upon gentle screen use.
So what about Samsung Galaxy S phones, do they shatter? NOPE, they use Gorilla Glass. So go ahead Samsung Galaxy S owners, drop your phones, your screen won't even scratch.
http://www.youtube.com/watch?v=6xASJobjD68&feature=player_embedded
So lets see:
Samsung Galaxy S:
Faster GPU
Faster CPU
More Memory
Super Amoled Screen
Gorilla Glass
T-Mobiles Faster Data Network on Vibrant
T-Mobiles Award Winning Customer Service for Vibrant
Super Light weight
One of the thinnest if not the thinnest Smart Phone
Evo:
Slower Cpu
Slower GPU
Horrible Graphics
Sub Par Screen
Screen Shatters easily
Horrible Customer Service
4G thats actually slower than T-Mobiles HSPA+
Bulky phone
Heavy Phone
FFC.. okay thats 1 plus
Kick stand that tilts over if you try to use your capcitive touch screen
Also the Epic 4G will have a keyboard. So if you don't like the keyboard, DONT EVER USE IT. Because the EPIC 4G will still be lighter weight with a keyboard compared to the EVO, and still basically the same thickness.
Once Again, I know someone is going to bring up the GPS issue. Samsung has releases an official statement. The GPS issue is going to be fixed with a soon to be released update.
Again, Sprint will soon have the Epic 4G.
Update: Epic 4G unboxing video
http://www.youtube.com/watch?v=060L_OZFe-M