Zipper vs. Non-zipper Boots
Based on cranetruck's and jasonjc's response I am going to change my position slightly, as I agree with both of their positions (no, I'm not running for office).
Cranetruck is spot on in that if you run your trucks regularly and KNOW what's going on up front, then use the USGI zipper boots but make sure they're sealed up well.
My original response made the assumption that jasonjc's logic (and my logic) follow; that is, if you're replacing a boot you're doing it for the first time and have never seen the inside of your front axle's knuckle. My experience is, more often than not, MV hobbyists buy a truck, at some point replace the boots, and before those boots need to be replaced again that particular truck gets sold for one reason or another.
My point is to simply error on the side of caution; if you don't know PERSONALLY what's going on inside that knuckle, go through it yourself and know what's going on. Again, that component of your truck has a LOT to do with determining WHERE your truck will go.
Thanks cranetruck and jasonjc for chiming in!
Joe