@Markerrag -- A sewer is always preferable, but there are times when hooking into one simply isn't possible. You might buy a house in a new subdivision out in the county rather than in town, build a home in a rural area, etc.
Fortunately, septic tank technology has improved and maintenance isn't as big of an issue as it once was.
Oh, and if you do have a septic tank, make sure to use one of those products that you flush down your toilet regularly to break down waste in your septic tank. Some of that stuff can save you a lot of trouble.