My philosophy is that before I spend $20 on a disc I might not like, I'll try the $8 version of the same disc. If I like the cheap one, I'll move up in quality. If I don't, I'm only out $8. Of course, there are some discs that only come new in the $8 version (e.g., Gazelles), and I've always got one Gazelle in my bag.
Basically there are 3 types of plastic. The "regular" stuff, which is the cheapest stuff to buy (generally $10 or less) and is called D or DX among other things. Then there is plastic called "candy", which is usually translucent (semi-see through), very durable and pricey (generally under $20) Candy is called Z or Champion among other things. Then there is the "candy blend" plastic which is usually not translucent but still durable and pricey (again under $20). Candy blends are called ESP or Star among other things. The candy blends are not quite as durable as pure candy but many players like the grip better (a bit tackier).
Someone can give you the technical names of the plastic types. I have heard them but don't recall since most golfers don't use scientific language when there is good frisbee slang available.
If you would look in the bags of most tournament players you would find most of the drivers are of candy or candy blend, most of the putters are regular plastic and the midrange discs are a mix with candy/candy blends being the trend.
It would be great if the most durable discs were also the grippiest (tackiest). But generally regular plastic is the grippiest but least durable. Generally pure candy is the most durable but slickest. Slickness is less of a factor for drivers since you squeeze them hard when you throw hard. Durability is less of a factor for putters because you don't throw them as hard and they last a long time anyway. Grip is most critical for putting since you are not squeezing hard on short shots.
The one wild card in plastics is FLX. FLX is a new, different candy and so far only used by Discraft. It is durable but grippy and originally designed for cold weather use. Some players like it year round. I like it in cold weather best.
Candy and candy blends are amazingly durable. It may take year or more of regular use before it starts to break in. This is hugely advantageous because you will learn how the disc flies and it will not change. The more consistent the disc is the more you will gain faith and confidence in it. Basically a candy disc will last you until you lose it.
To decide which disc in which plastic you prefer is half the fun. Borrow friends discs and try them out before you buy blindly unless you have great faith in a recommendation. Used candy discs are a good deal. New ones may be shiny and clean but used ones fly just as well.
Keep in mind that a disc which does not work for you today may be perfect down the road. As your game improves your strength and form will improve, allowing you to control more overstable discs.
I try to stay away from D and DX plastics for drivers and midranges, for the reason Mark gave above. Star feels great in the hand and is nice and tacky, but is expensive. Elite Z and Champion takes AGES to break in (lot longer even than Star and ESP), but once you do break them in, you get a long life out of them. Ask your friends what plastic they throw, and ask if you can feel the different types. Really, its an individual preference thing based on your budget and what you like.