I cant comment on where is the best place to get an iPhone from on Vodafone as I dont know to be honest.
What I can comment on though is the Apple Vs Android debate as I have used both extensively.
Which you prefer is largely down to what you are going to do with the phone.... (if you are just going to make calls then dont get either....
)
I had the HTC Desire on Android and I loved the little things that it did as standard, although I believe some of this was actually down to the HTC Sense overlay rather than the pure Android OS.
When I went onto the iPhone there were many of the little simple things that I missed from my HTC, some I still do!
However, if you are an apps junkie, particularly for games then there simply is no contest. The Apple app store wipes the floor with Android in terms of content.
There maybe more free apps on Android, but for strength in depth and quality of apps then it has to be Apple.
However, there are also many reasons not to buy the iPhone. No Bluetooth, no external memory facility, extortionate fixed tariffs compared to similar Android devices, hugely overpriced handset again compared to Android....
I am currently using the iPhone 4 and I love it, but to be honest if HTC sorted out the pathetic internal memory capacity on their phones then I would go back. Not until they start including at least 8gb as standard though.
My HTC Desire became useless. I filled the memory within weeks, so I moved what I could to the SD and deleted the rest. Within 6 months though the phone just accumulated so much background junk that it was permanently full and stopped simple things like emails downloading from working. I tried for ages to resolve this, and consulted many 'experts' but the bottom line always came down to needing a factory reset and losing anything that I couldnt back up to SD.
So I did the reset and promptly sold the phone a few days later!
So I would say to think what you want the main feaures of your phone (calling aside) to be and then choose the OS that best suits your needs.
If that happens to be the iPhone then unfortunately its just a case of paying the price..