What should I do about my phone contract?

My phone contract for my Samsung Galaxy S6 ends in late September next month. Iceberg been using this phone for the past 2 years. My dad says that I should stick with the same phone but get a sim only deal with plenty of mobile data. The thing is, I would but the S6 has such a terrible battery life even if I use it in power saving mode. I was thinking of getting one of the 3 new iPhones coming later this year. But that would be way too much money, so I'm thinking maybe I could find a good deal for iPhone 7 Plus and if I can't get that maybe a OnePlus 5 and if I can't get that I'll compromise and get an iPhone 6s Plus. I use O2 (UK carrier). What do you think I should do?

Depending on how you use your phone, the iPhone might have terrible battery life, also. If that's really your reason for getting a new phone, it may be better to simply start using an external battery.

If you no longer want to be under a contract I would say once you pay off the bill unlock the phone and sell it to someone else and save up a little more to buy a better phone. If not you can upgrade your phone under the contract but I believe you would have to extend the contract time.