Moderator Post (s) for this thread: >Wooden Guttersbythiggy on 01/10/2011
Aluminum dents, and plastic can crack in cold weather. Both expand and contract with heat and cold so they have to be installed properly. Plastic can weather badly if it's not a good virgin plastic or improperly made for any reason. That you won't know for several years probably.
Generally aluminum should cost less to make and install, but that can vary widely.
Your best bet is to deal with a local company that has been doing gutters and downspouts for the last 30 years or so. They have a reputation to uphold and generally if they have been in business that long their guarantee means something. If a company gives you a lifetime warranty but goes out of business it is worthless.
In the northern areas of this country aluminum or even galvanized steel are common because of the freezing temperatures.