Exterior siding offers extra protection from wind and rain. It also gives you a surface that is easy to paint or that comes pre-painted. You can get exterior siding in a primed version, too, with all of these depending on how much work you want to do yourself. If you don't have a preference for how much painting you might have to do, look at the color you need to get and go from there.
Matching the Rest of the Neighborhood
If you live in a subdivision run by an HOA, you'll need your house to match the color scheme determined by the board. You might have a few choices of siding color but will have to stay within that group. Chances are pre-painted siding will be the best choice for you because then you can get the color you need without worrying about the hue being off just a bit, as can happen so often in painting.
Getting It Over With
Maybe you don't have a preference for whether or not you'll have to paint, but you might still want to just get everything over and done with. Pre-painted siding would be perfect for you if you could find a color you're happy with; given that many siding companies can customize siding colors, you've got a good chance of finding the one you want. If you can't, you can always get factory-primed siding, which has priming paint on it but not the final layer that adds color.
A Little Customization
Factory-primed siding is perfect for those who don't fancy doing an entire paint job themselves but who would like a higher level of customization than what the factory can provide. You get to choose the top-layer paint color and can go have a paint store make a customized mix for you. You can also hire a painting company to do the job if the siding company doesn't offer painting services.
You Live for DIY
Of course, you can also go all out and get plain, unfinished siding that you have to prime and paint on your own. If you happen to like these huge DIY projects, then getting plain siding is best for you. Just keep in mind what a big job it is; you may want to hire a painting company to do the painting instead of doing it yourself.
Check out the online galleries and catalogs from the siding company to see what's available. Companies can order painted siding in so many colors that you may end up choosing a pre-painted version anyway to save yourself trouble and time.