Posting this in an iphone owners forum probably means you'll get opinions more biased to the iphone, however for what it's worth I'll give mine. To lay my cards open I own a Samsung Galaxy S and got an iphone 4 for xmas which I'm selling basically because I don't need 2 high end phones, I can get more money selling the iphone and frankly, after owning Android phones for 2 years I prefer it to the Apple IOS.
However, the first key thing I noted is that you stated a preference for the iphone, so on that basis I'll recommend you get it if you can afford it, the hardware's well built, the accessories available for apple products is simply unrivaled by any other out there and the phone simply does what's on the tin. In my opinion the iphone offers the best balance of functionality, design, battery life, and ease of use.
The main downsides where I feel the SGS has an advantage are firstly price, for all it does the iphone no longer offers a unique experience so the premium you pay for it is no longer justified IMO, i.e., I could understand the cost in the early days where credible rivals were lacking but not now when some are now arguably superior.
Secondly the 'apple tie in'. It's frustrating that apple doesn't share the same open standards as other phone manufacturers, so things like chargers & TV out cables must be apple specific, whereas on my SGS I can use the TV out cable from my nokia N95 and any other micro USB car/home chargers from other phones. The close integration to itunes isn't to my taste, once my favourite podcasting software, it's become very bloated over time and one to avoid IMO! You want to travel abroad and pop in a foreign sim or want to use a differnt handset briefly... well you have a microsim in the Iphone 4, still possible but a bit of a hassle.
Apps Market - Some may state this as an advantage but for me the control apple has over apps it allows into the market is too heavy handed. For example in the Android market there are a plethora of music download applications that will give you music chart listings from 1960 to date and allows you to download any for free; the legality of the downloads is questionable... but the point I'm making here is that you have that choice on the android platform, Apple would never allow that and neither does it allow adult content. A further example is that you'll find tools which enable you to hack your phone and make fundamental changes to it legitimately on the Android market, so if you're techinically minded and enjoy such pursuits you'll find the apple ecosystem restrictive.
Finally due to the multi vendor approach to android i.e you have Sony Ericsson, Motorola, Samsung, HTC to mention a few of the Open Handset Alliance members developing on Android and each adding their customisations, there's a wide range of stuff ripped from other phones that you can use on yours, so if you fancy a widget or keyboard from motorola Droid, or HTC Desire HD, you may be able to install on your android phone and build a completely custom system.
As much as I've tried I cannot assure you my views are balanced due to my my long association with Android, however I wanted the iphone 4 when it was announced but got the Samsung instead, however now that I got the iphone 4 I'm happy to dispose of it without hesitation or regret. Not that the Samsung is troble free, it does has a serious battery drain issue and video quality on it isn't up to the iphone's standards.
Good thing is you're considering 2 phones any of which you'll be happy with. In summary the iphone is a more controlled phone with brilliant build quality, relatively decent battery life and does everything well. The Galaxy is more exciting, a lot more open to configure and customise, but requires more maintanance and time to setup to your taste.
In the early days of the Samsung I know a few people who swapped it for the iphone 4 because the initial release firmware was very bad , now I notice a reverse trend with people replacing the iphone with the likes of the Desire HD.
Iphone/Android discussions get a bit touchy and I hope this doesn't decend into a heated discussion, so for that reason I won't make further comparisons. If you want to ask specific questions about the Galaxy please post in its sub-forum and I'll be happy to respond.
Good luck.
FF
