The Decision of Buying a New Phone

Ahh the decision of buying a new phone. My current phone is a Nokia Lumia 520, and due to some issues, I think it is time to upgrade. So I've been thinking. Windows 10 Mobile is coming soon, do I want a phone that will support it. There goes my phone. So I've narrowed it down to three phones. Lumia 550, 640, and 635. All are good phones, and each has its pros and cons. Microsoft categorizes it's phones into three categories. Affordable, midrange, and high end. The phone I currently have is affordable, so I want midrange, so I have a phone that works.