There are different flavors of each type. All the way from uncoated nylon on one end to polycarbonate coated polyester on the other.
There are different weights of materials available as well. Icarex is about .6 ounce per square meter, not counting the coating's additional weight. Some folks feel white is lighter in weight than other colors because it's not carrying the dye. Dacron on the leading edge sleeve is 3.9 ounces per sq/meter. Banner fabric is usually 1.5 ounce nylon.
You could buy a kite made out of polyester fabric, but one crafted of Icarex will certainly cost you more. Some materials are laminated together to incorporate multiple objectives. Scrim in the mylar, or a mylar back nylon, for example.
Poly doesn't gain weight as quickly when wet compared to nylon, nor does it stretch and return to shape. Surely you could see applications where both of these treats are desirable. How about in competition and you want the kite slower?, wetting a nylon kite is not news! Or you're just jacking around in bad weather, chasing seagulls on the boardwalk, then a more responsive poly kite is called for.
Orcon can not be sewn or come to a sharp right angle in construction,yet it has uses for indoor kites despite these limitations.
Many kite builders in my club begin with tyvek (like a Fedex envelope, synthetic paper) prototype. It might be stapled onto a dowel wood frame or duct taped together. The kite will explode and the stress points can be identified, subsequently reinforced and tested again with a more complete prototype.
Different fabric colors can be combined, but mixing fabrics is a no-no for most builders. Some of the best do it anyway, it's all about color selection. If you needed 12 purples on your design you will have to mix fabrics. Different stretch and bias characteristics will need to be thought thru and planned for.
I'm no scientist, but some materials are worth more time to use and funds to acquire in my experience. I live in the land of no-wind, I'll take catastrophic failure over stretch anytime.