Is the 640 a good phone?

I feel like I shouldn't be even asking this because its like 40 dollars on ebay I could just try it out myself. But the reason im asking is that I heard you can run either windows 8.1 or windows 10m. And I want to use both here and there so is 640 the best option the highest end option that supports both windows 8.1 and windows 10 or would you suggest something other then that?