Screen size is second to screen quality, i am not just talking about ppi, on 640 xl when the screen is off, you can actually see blocks n row/column pattern on the display. Before you buy anything, get a hands on ur self first
My opinion is Lumia 730. I had to decide between both myself. Lumia 730 itself feels way better in the hand. The screen is amazingly bright as it is oled. Not to mention the ppi. Ok the 640 xl is way bigger but I rather have more definition with the 730 and not see tiny pixels when reading text.
Although I possess a 730,I honestly have to tell that the 640 surpasses the 730,it has glance screen,Lumia Moments,13mp camera with Zeiss optics,bigger battery.Not to blame the 730 because it was launched at a different time scale,if you are a avid fan of Nokia then you can buy on of its last Lumia phone.But personally using a 730 I would definitely tell you that buying it is a good investment.