The retina display is the main reason to buy an iPhone 4 rather than a 3GS. A few weeks ago I took my 3G and restored it to clear it for sale on eBay and I couldn't believe the difference. If you've used an iPhone 4 then look at a 3G or 3GS the thing is going to look like crap. Not to mention the slimmer design, better camera's and better WiFi
I've used all the major devices like the Droid X and HTC EVO and I still won't leave the iPhone. Some of the features are nice with Android but nothing you can't do modding the iPhone. I never understood the whole Android is more open than the iPhone too, since anything useful like free tethering has to be hacked on either device. I'm still in the debate about the upcoming HTC Thunderbolt and if it's worth the switch for LTE or maybe even the Verizon iPhone 4. Guess I'm just waiting them to announce the plan details before I consider either option.