Also considering that the 640 XL is going to be around $200, be aware that $200 is also the common price of a 1520 in excellent condition these days...
The 1520 is the better device by far
But with the 640xl your getting a device that is much newer, and you'll be covered in case of defect.
While with the 1520 you'll have to be lucky to get a red, white, yellow, or black still in warranty, but the green model (released this past fall) hasn't even been available for sale a whole year yet, so those will all most likely be in warranty, which will be a big bonus in case you encounter "ghost taps", but as long as you don't put your 1520 in your front pocket then you'll most likely never encounter them
1520
-same price while being better in every way (unless of course being .3 inches bigger is uncomfortable for you)
-Will need to get green in order to have a warranty
-will be Used unless you want to dish out an extra $50 for a brand new one
-you'll need to keep it out of your front pocket
640xl
-newer so you're guaranteed warranty & support
-more comfortable size
-lower specs and quality ( judging by the fact that it is a budget device)