Just wrote a very long reply and Chrome for iPad lost it - not sure which company to blame for that
Anyway, thank you all again so much for your responses, a friend kindly let me test run his S3 for a while earlier, and...
Things I liked:
+ Cost; factoring in micro USB and micro SD slot, I'll save almost £200
+ Size; extra width allowing a 720p resolution is nice
+ Widgets; very useful, with a nice app list to fall back on
+ SwiftKey; for showing the kind of customisation that simply isn't possible on iOS
+ NFC; for showing that Samsung are trying to stay ahead of the curve
+ Toggles; so simple, yet so essential, requires jailbreak on iOS
+ Speed; it felt really quite nippy, especially the camera
+ Messaging; messaging is a truly awful experience in iOS without jailbreak and BiteSMS
And some things I didn't like:
- No PhotoStream; no way to easily and instantly sync to iPad and Apple TV
- No AirPlay; very useful for Plex, iPlayer, Sky Go and gaming
- Gaming; no GameCenter, limited choice of games
- Apps; repurchasing apps will be costly (LogMeIn alone is £20)
- Play Store; limited availability compared to iOS, no way to backup/sync to desktop(?)
I'm almost tempted to wait for the Samsung Nexus 2, though none of my negatives regarding the S3 will be fixed by doing that.
It will make my experience very fragmented though - no seamless sync between devices, no AirPlay, no GameCenter, having to repurchase apps I already own, as well as potentially buying future apps on two devices too, hmm.