I am no expert, of course (disclaimer aside now) And of course you should do a search of past posts on these materials.
There, now you should get nothing but good info!
Micarta is tougher, absorbs few chemicals, you can put a lighter to it and it will take a while before you wreck it (It may blacken but the handle will be o.k.), you can get a myriad of colors, blah blah blah.
Zytel is cheaper plastic. Still good for Every Day kinda work. The micarta will be longer lasting ad is more beautiful. And if it is bead blasted canvas micarta and your hand is wet, it seems to me like your grip IMPROVES!!!
Zytel is o.k. too though, don't get me wrong! You will probably see zytel only on production knives though. I think it is injection molded where Micarta is machined.
We talk about Zytel as if it is some cheap plastic, but it is actually FRN -- fiberglas reinforced nylon -- and can be pretty tough. If it is made in thin slabs on a small knife it will be flimsy. I just got a Cold Steel large (5") Twistmaster and the Zytel handle is a thick barrel, massive and durable, and a great grip. Of course, it is inexpensive and lightweight and easy to mold, so it holds down the cost of a knife. I like that.
Micarta is a laminate, alternating sheets of a fibrous material like cloth or paper and an epoxy binder. It is very tough, looks good, can be made in layers of alternating colors almost like wood grain. Feels good, grips well.
Micarta handles are indeed very beautiful. So, for a collector, the choice is obvious.
However speaking of hard usage... Hm, I don´t know guys but I´d say that Zytel is the more practical one. Let me explain my point of view:
1. Zytel is more lightweight
2. Zytel is cheaper to manufacture (and replace if damaged)so one is able to make a step up in steel quality
3. In opposition to Micarta, Zytel does not expand or shrink in cold, warm or moisty (wet)conditions.
The last issue is in my opinion the most important one. As speaking for myself, living on the Swedish west coast, the weather around here is often discusting, so my choice is easy. Also during winters, the temperature sometimes drops way under zero (Celsius). In an anvironement like this, Micarta easily cracks due to the strain.
I don't think there is anything inherently wrong with Zytel aside from it's plasticky look. But then again, the current benchmark synthetic, G-10 can be homely in its plain black guise too (but have you seen the new CRKT WASP?).
Back to Zytel: just because it is inexpensive to manufacture does not demerit its functionality. I've said many times that my childhood BMX bike -- a Redline! --had Zytel mag wheels, and seat and I abused that thing more than I could, or should, a folder. It took it all, and then some.
Micarta, and G-10 are a bit more exotic and "cutting edge". Therefore they are valued more.