Well, I'm ready to purchase the Galaxy S4. Being that our son works handling the premier accounts for a major phone provider I get to test out many a mobile phone. His personal favorite and the one he uses for business is the Note 2. Apple has a lot of catching up to do if they want to be more than just trendy. He told me yesterday that even before the S4 was announced that his customers were calling him incessantly to see if they could get one. Surprisingly, he also said that there is interest in the new Blackberry. Now, before you say he's biased, I can tell you he's sick of talking about mobile phones and dreads me calling him to talk about them.

I'd take the S4 over the Iphone 5 for the bigger screen. Not JUST because it's bigger but because I am a guy with large hands. Trying to type on a touchscreen is murder on smaller phones. The fact it's a Droid is icing on the cake.
On a side note, I got to play with the new Blackberry Z10 for about 2 hours today. It has a nice mix of features and while I'm not in love with it, I don't hate any one thing, other than a lack of a home button. If you are a Blackberry user, and have been holding out. Wait no longer. You're upgrade is here.

I'd wait for the iPhone 5S especially since it might be coming earlier than expected. It is rumored to have a lot of interesting new features such as fingerprint scanner, iOS 7 which is rumored to be completely new and finally NFC. And by the way these are features that count and are useful. The S4 was somewhat of a disappointment for me what with it's new useless software features such as Smart Scroll. It's not revolutionary but just an evolutionary update with the camera and processor updates. And proof that Samsung disappointed is that Apple's stock prices shot up after Samsung's announcement of the Galaxy S4.

The GS4 is made out of polycarbonate; a type of plastic. Calling it plastic sounds off putting. It'd be like saying the iphone5 is made out of soda cans.

Total Apple fanboy comparison. He left out NFC again like he did and was called on in the S3 comparison. The SD card is a huge advantage with high def video capture yet that's glazed over as well. Apple makes a good product, but it can't compete on features.

One BIG aspect of battery life that wasn't mentioned is how ridiculously easy it is to pop-open the S4's rear cover and insert a fully charged replacement.

it's odd that Samsung gets picked on for choosing a plastic case.
You have to buy an after-market plastic case for the iPhone so it won't shatter as soon as you drop it. That makes it thicker than any other smart phone.
Aluminium case might feel nice, but it's no cushion, and it transmits too much force to the screen when you drop it. Yes... when not if.
There are plenty of side-by-side drop tests of Galaxy x vs iPhone y.
these reviews should be done with the bump case attached, to get a real-world experience comparison.

@Nirmal
Your post said you are waiting for iPhone 5S because NFC counts and is useful. Samsung has had NFC since S3 and iPhone 5 doesn't have it.
The hardware is updated but some of the software updates look cool too. S Health used the acceleromiter to work as a pedometer kind of like fitbit and fuelband.
Knox lets you create separate sandboxes/profiles for personal and work. It lets you separate your work/personal contacts easily, stick your work mail in the work sandbox, and if your IT department needs access to remote wipe your phone they can wipe just that portion. It is useful for people who were previously carrying around 2 phones. This is something BB10 was going to support and now it's available on S4.
It will come with SwiftKey built in which is probably the best Android input app.
The iPhone 5S won't be out for a while so the Samsung will have the dominant product for 5-6 months or longer if the iPhone 5S doesn't deliver.
It is evolutionary over the S3 sure but most S3 owners are still locked into contracts anyway. If you compare it to the SII generation phones it's pretty significant.

About the mass... The Samsung Galaxy S4 is relatively lighter:
S4: density = g/mm^3 = 130/(70*137*7.9) = 0.00172
iPhone 5: density = g/mm^3 = 112/(59*124*7.6) = 0.00202
So, the S4 is less dense, and if it had the same dimensions as the iPhone 5 it would weigh about 95.4g, about 15% lighter than the iPhone 5.
Relative mass matters, not the mass itself!

everyone states that the S4 is 'Plastic', Asthough it is some cheap household plastic which degrades the phone somehow...
It is polycarb plastic. Extremely durable, Scratch resistant. It doesn't dent or mark like aluminum. You can bend the back cover in half so the top and the bottom touch each other. It'll ping back to its original form without causing any issues, stretching or tears. It's an expensive plastic too!!
Least I add, It doesn't get really cold and uncomfortable to hold in cold weather like the I phone!!
i wish people would get educated on the subject before dismissing something due to ignorance. The next Iphone should be polycarb, it'd make it a half decent phone just for that alone.