£699 for the new 64GB phone is a lot of dosh, particularly since it’s a pretty sure bet that there will be an iPhone 5 in a new body within twelve months. The 4S starts at £499 for the 16GB model and, probably, the sweet point will be the £599 32GB phone.
My current iPhone 4 has 32GB and I have never exceeded 16GB in store. So I feel fairly confident in recommending another 32GB version to myself. Between now and Friday, though, I need to find out whether iCloud, Siri and other enhancements will take up significantly more storage. Maybe there is a case to be made for the very expensive 64GB 4S but I don’t see it yet.
Whatever flavour you choose, please bear in mind the excellent resale value. So far, iPhones have commanded amazingly high secondhand values and you can realistically expect to get half your investment back after twelve months. That eases the pain, assuming you have the cash to go for an unlocked phone in the first place.