I have an Iphone 4, Captivate, Droid Incredible, and another Android phone I am too embarrass to mention. I often have 3 phones in my pocket.
The Galaxy S/Captivate is the first Android phone to really challenge the iPhone 4. Seriously. It is uses top-notch gear and internals. For example, it has a Wolfson DAC which makes its audio playback on par with the 3Gs. The screen is gorgeous but all AMOLED/Super AMOLED screen suffer from Pentile Matrix distortion. The Screen on the iPhone 4 is stunning. Other than the screen, the build quality on the Samsung is really good and the hummingbird processor is fast.
They're ALL good devices. It boils down to your preference. I swap my SIM between my iPhone and Captivate all the time depending on my mood.
I continue to use the iPhone for my work. Nothing on Android can do what I need to do my work. Period. I also have some specialized iOS apps that I use on the phone on a regular basis. The Apps is what is keeping the iPhone alive. I also love the battery life of the iPhone and camera. I can go 2-3 days with only a minor 15-20 minute top-off charge in the morning on the iPhone. I also don't have to run task-killers, disable notifications, wifi,bluetooth or gps. Power management is iOS forte. Battery life, there is simply no comparison.
But I do like the Samsung phone. It is swhuuuuuueeeet.
The only fear I have is Samsung is notoriously bad at shafting their customers with updates and supports. I am taking my chance with Samsung so take my recommendation with a bit of warning.
As for the antenna issue, I know 30 people w/ iPhone4 and none of them have reception or drop call issue. It effects like .05 of the most vocal users and ihaters.