Typically, gas grills have either cast iron, porcelain coated cast iron, stainless steel, porcelain coated steel or just plan steel. Now the intense heat of the grill can wear down these metals if they are thin or poor quality. Actually the worst thing you can do to a cooking grate is heat it up and then throw cold food on it, but that's grilling. It is important to keep you grate clean and in the case of bare cast iron to keep it oiled to prevent rusting.
Plain steel tends to lose its temper and start developing chips which make the surface rough and uneven. This leads to sticking. The plain, thin rod type grate is bad so stay away from it. Stainless steel can also lose much of its non-stick abilities and get chipped up but not nearly as fast as the plain steel units. Stainless steel will work well for a long time, but not as long as cast iron or porcelain coated surfaces. Porcelain on the other hand has a nasty habit of chipping away and exposing the metal underneath to moisture and can cause rust, especially in porcelain coated cast iron. This isn't to say that porcelain coatings can be bad, but cheap porcelain coatings can be. I you do have a porcelain coated cooking grate you need to keep hard metal scrapers and tools away from it. A metal bristle brush is okay, but no scrapers.
Now I prefer bare cast iron, and heavy grates at that. I like a surface that not only gets hot, but holds a lot of heat. It gives good grill marks and helps get food cooking faster. Of course a bare cast iron cooking grate needs to be taken care of. This means keeping it clean and making sure that it is well oiled over the entire surface. During grilling grease and fats will build up on the surface of the grate and this works to help protect the grate, but you don't grill on every single square inch of the grate so you need to take them out frequently and make sure there is no rust and that they are well oiled. One of the problems a grill presents to cast iron parts is that the intense heat causes grease to vaporize and therefore burn away. Just because you grill fatty burgers all the time doesn't mean that you've got enough oil in the right places.
A good, heavy cast iron grate should last for decades if you take care of it. If you simply are not willing to do the work, go for a high quality porcelain coated cast iron grate. You get the heat characteristics of cast iron in a rust resistant surface. When you buy your cooking grate though, get a good grill cleaning brush that won't chip the porcelain.