My brother has the Intellimouse Optical, and I have the Explorer 3.0. I really like how the Explorer is angled so it fits your hand. Naturally, you had doesn't lie flat, it lies at an angle (put your had down naturally and notice the arc under your knuckles). In that respect, I like the way the Explorer feels much better. I relatively small hands, but the Explorer fits them perfectly. I don't know if it's just because I've been using the mouse for so long, but if I let my hand assume it's natural position, then place it over the mouse, everything is exactly where it should be.
The Intellimouse Optical is also a good mouse... I just like how the Explorer feels much better. But, as Jittles said, go to the store and feel for yourself, as mice are all about personal preference. I'd get an MX700 if I was getting a mouse right now, but that's a different story...
Squibby