i've been using the iphone since it came out (now using 3gs with an iphone 4 that just went off backorder), but i also just bought a nexus one to try it out and see how i like it. if i end up liking the nexus one/android better, i'm gonna sell my iphone 4 to offset the cost. if not, i'll return the nexus one and keep the iphone 4.
after a couple of days using android 2.2 on my nexus one, i'd have to say that in my opinion android is currently a couple notches above iOS. the notification system is such a pleasant experience compared to iOS' intrusive popups and pretty much everything on android is modular. meaning that you can easily customize/replace many parts of the UI (custom keyboards, launchers, etc). unless your phone is locked down by a carrier, you have the ability to install any 3rd party app you want, which is really nice. apps have a lot more access to the OS and I love how all my google accounts are so easily integrated into everything. the built in wifi hotspot and usb tethering are independent of the carrier, so that's also a really nice feature.
however, with all that said, i am currently leaning towards keeping the iphone 4 (pending on how badly the signal/proximity issues affect me when it arrives and seeing how i feel after using android for a bit longer). why? one single reason - apple's app store. at this point, i'm simply too used to and attached to the apps we have available on the app store. the polish, utility and the games can't be beat right now. while the android market has alternatives to pretty much every app I use on the iphone, sometimes it's hard finding apps with the same quality and polish. the games in the android market are also currently lacking.
maybe sometime in the future when the android market has matured more, i'll be able to switch platforms without any doubts, but in the meantime i'm probably sticking with the iphone.