I've got a HTC Desire, and I love it.
Having said that, I've played around with an iPhone and they are a nice bit of kit. The Android OS just gives you a lot more flexibility and customizability. (you're not tied in to a specific browser, or messaging app)
For me, the thing that the Android does brilliantly is seamless syncing. I've used Google for email, contacts and calendar for ages, and now whether I make a chnage on my PC or on my phone it updates to "the cloud" instantly. This means that when I upgrade my phone all of my contacts get imported automatyically when I set it up. No more tedious manual importing. And the Gmail app is great.
If you don't use Google then it will still sync to your Outlook, but you'll need to plug it in to the USB to do that.
If you're slightly techy, you should definitely go for the Android over the iPhone. If you're not, then I'd still recommend it, but you might want to try and use both before you commit in case you do prefer the iPhone.
Android 2.2 supports full Flash, but even on the older versions you can play YouTube clips through the official YouTube app.
For music, DoubleTwist is the canines cojones for syncing to absolutely anything. It can even use existing iTunes playlists.
As already mentioned, HTC have a great feature where your contacts are linked to Twitter and Facebook, so you can quickly see updates from a specific person. Not sure how useful that will be for you.
For cycling, there's an app called MyTracks which is brilliant. It logs your route with GPS, and uploads the stats to Google Docs and plots it on Google Maps too. Look in my sig for details.
The only downside, common to many modern smartphones, is battery life. If you have it set to sync data permanently you'll need to charge it every night. You can eke this out to a couple of days by being more selective with data use.
Let me know if you have any other questions.