Since they are in and around the same price, wouldn't the explorer be a better purchase?
Logitech's specs for it's MX line are as follows:
Image Processing: 4.7 megapixels/sec
Resolution: 800 dpi
Acceleration: 10g
Speed: 40 inches/sec
I really don't know how it compares to the Explorer's 6000/sec scan rate, or whatever. All I know is that the MX500 works flawlessly. I've moved it around as fast as humanly possible, and it never screws up, and maintains complete accuracy. This is probably the case for the Explorer as well, as optical mouse technology has just gotten good enough that there aren't any problems (you still have to buy the GOOD optical mice, though!).
I say, go with the one you think looks/feels better.