Should I sell my phone to buy one second handed?

So, I've had my iPhone 6 for a few years and it still works okay apart from the battery, but I can get that fixed for $30 so whatever. But then again, I could sell it online for 175 and buy an iPhone 6s for that same price. But second handed. The thing is, I'm not sure if I should. I really want a new phone but what if I buy one and there's something wrong with it? Should I just take the risk or stick with my old one?

There isn't a whole bunch of difference between the two devices you've listed, apart from that if the old one is nicer then go for it.