This is a really hard one for me. I shall now bore you with why
I've tried a few Android devices, and they all were uniformly awful - mainly down to the OS itself rather than the hardware (The Galaxy S was awesome, if plasticky!). It seems that Android world is all about features, numbers and specs, and gives no thought at all to the experience of actually using the phone. And for all their talk of openness, you're remarkably tied to the services of a company who's only interest is serving you ads, and all the profiling and tracking they do in order to do so. Just one example - try syncing a calendar with anything other than Google Calendar and see what I mean. Bleh.
I've had a couple of iPhones of various vintages (including iP4), and enjoyed their amazing slickness, attention to detail, and app support, the product design, how nicely it all works with my Mac, etc, but the incredibly intrusive notification system eventually drove me away.
I have to admit, since I got my HTC Trophy in April, WP7 has surprised me. Really glad that I was pragmatic enough in to have looked beyond Microsoft's awfulness elsewhere (ribbon UI fail, anyone?) and in their dodgy mobile OS past. They've done a sterling and original job of it, and the interface is just lovely. It is incredibly clear that the designers have thought very hard about how people might use a phone - things like being able to take a photo without unlocking the phone, "gentle" notifications, easily the best soft keyboard and having calendar reminders visible in the lock screen have transformed the smartphone experience for me. I don't know if this way of describing it makes sense to anyone, but the WP7 experience is <i>quiet</i>.
Realistically, I will be tempted by iP5. Things like not bring reliant on three separate parties for OS updates (Microsoft, HTC, Vodafone in my case currently), that amazing product design and polish, and predictably great hardware to boot.
For the first time ever, it isn't going to be straightforward. It will be really tough! Lets see what October brings. Should be an interesting, fruit-themed month one way or another!