The 420d is the old standby 7-8 oz packcloth - smoother texture than the cordura but still very durable. The 1000d cordura you are probably familiar with, but just in case, for comparison the 500d cordura is about 8 oz/sq. yd., while the 1000 is about 11. The 600d polyester is almost certainly faux cordura - that is, a textured fabric that looks like cordura but is not quite as tough. For your purposes (tough gaiters) all will do the job. The 100d will be the toughest for sure, but even going down to 330d you're getting some very tough fabric. The difference in weight will be a couple ounces or so, so it isn't a lot. if you want the absolute toughest, get the 1000d, if you want light but still rugged get the 330, if you want a compromise get the 420 or 600.
One minor factor is that the polyester will hold up to ultraviolet degradation better than the nylons will - but that likely won't matter for your use.