Im not really an expert on the aspects of leaving a gun in a vehicle, but Ill take a stab at in anyway. First, I think that if the gun is well lubricated (using high quality lube or grease) it should withstand the temperature extremes. However, the results may vary if you keep the weapon in a holster. For instance, its my understanding that leaving a weapon holstered for too long can invite oxidation. I believe this is due to the fact that the holster can strip away and/or absorb any lubricant necessary for maintaining the weapons finish.
If you plan on keeping this thing in your vehicle all of the time, it probably would not be a bad idea to keep an eye on it, in terms of any visible effects of moisture. Plus, Id probably keep such a weapon maintained (cleaned or at least lubricated) on a fairly regular basis and do regular function checks. Ditto on making sure the ammo is in proper condition. Personally, if moisture is of any possible concern here, Id keep the ammo (at least) indoors. If we are talking about a semi-auto weapon here, this should prove to be no inconvenience at all. This, aside from the benefits to your ammo, would also keep you from having a loaded weapon in the car upon your absence.
In terms of which finish would be most durable...I cant say with any "facts" to back me up. I cant vouch for temp extreme resistance, but I like polymer weapons because there is obviously less metal to oxidize. Plus, I think they are more resistant to blemishes vs all steel weapons. Either way, most high quality guns, whether polymer framed or not, will probably resist harsh temps quite well. Id personally be more concerned about "function" vs "finish" when dealing with this type of scenario.
Lastly, Id like to say that I personally do not condone keeping ANY firearm stored within a vehicle for an extended period of time. Not only are you inviting more "wear and tear" on the weapon, but it just makes the gun far too accessible to those who may misuse it. Personally, Id just keep the gun indoors and transport it as needed. After all, if it makes it as far as your garage or lot, why not just bring it inside?