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Vodafone Finds Brits Keep Mobile Phones for 4 Years Instead of 2
(www.ispreview.co.uk)
This is a most excellent place for technology news and articles.
Regarding flagship models: I found that previous years flagship or current mid-range with similar specs (I use gsmarena to compare) is a good compromise between speed and lifespan, and always go for more memory, today's android uses about 20gb so 128gb must be absolute minimum if you plan to have a few apps installed.