Will using a knife sharpener reduce the lifetime of my knife?
You should use both. Knives have a very thin ragged edge (the burr) that is too small to see, which gets pushed over from use, and which makes the knife seem dull. A honing steel straightens this burr out. I use a honing steel before I cook every day.

Over time, the burr wears down, or can no longer be straightened. At this point, you will need to use the sharpener, which actually removes metal from the blade. One metaphor would be that if your knife was carpet, honing is like vacuuming, and sharpening is like steam cleaning.