The iPhone 4s is a great phone but has older harware. Why not try get the iPhone 6? I'm not sure the phone will support iOS 9 due to its limitations. The one plus one is a good phone as well but it all boils down to personal preference.
I prefer iOS for daily usage but the 4s is up in age and has limitations. No LTE for example and its probably close to its last iOS updates. Its all personal preference but a very good
Choice would be a used 5s you can get for cheap if you're looking to save a little bit.