If I didn't have an iPhone, I'd have an android phone over a blackberry. I quite like the samsung made googlephone (google nexus s). The Sony opera arc, Samsung galaxy s and h&c desire range are all pretty competent phones. There's a whole breadth of née level droid phones coming out before too long as well I think.
I'd be questioning your o2 renewal offer, as if you go to their website, a new customer can get an iPhone 4 on 18mnth £37pm contract for £202 upfront, Or 24mnth at £32pm (or same monthly and lower upfront {£140}). Looks like they're nearly making you buy the phone outright and putting you on a high sim-free type tariff. (probably not, but it's not too far off). You might also want to wait a bit. iPhone 5 will be out before too long and they'll probably do the same as they've done with the 3GS & iPad 1 and knock the price down a little.
There's always another secret option. Stick with current iPhone?