What matters more than either is how well the car was taken care of. My current car is a 1999 with 154,000 miles on it at the moment, and runs like a top, but a friend of mine had essentially the same car but a 1995 with 114,000 and it was a scrapheap. Had nothing to do with the fact that it was older; it was just neglected and abused. Of course, at less than say 80,000 the consequences of neglect and abuse are not yet generally apparent, so you have to be even more careful.
Also, the type of miles does matter. 60,000 pizza delivery miles put a lot more wear on a car than 100,000 highway miles with the cruise control set at 60.
It also depends on the car. Some cars have model years that are known for being more or less reliable or long-lasting. If you're looking to get a Car X, but 2004-2005 models of Car X are flawed in some significant way, then a 2003 or 2006 is far more desirable.
Rust can be a function of age depending on where you live. Some cars are more susceptible to it than others, some years of certain models are more or less susceptible, but as a general rule of thumb, the older the rustier. However, a low-miles older car might not have seen much winter driving, so it could be far less rusty. You have to research and know what to check.
Also, people think of 90s cars as old, so a 1999 model tends to be cheaper than it should be by comparison to an essentially identical 2000 model.
Just another reason to do all your homework when it comes to buying a car. Do extra work upfront, and it minimizes the work after the fact.
