Yes, there usually is a noticeable difference in quality between a cheap Sigma or Tamron lens and a much more expensive Nikkor or Canon equivalent lens.
However Sigma do make some decent lenses so it's worth looking for some reviews and comparisons online.
For an amateur you might not mind the lower quality lens.
Barrel distortion for example isn't always that easy to spot unless you're taking photos of geometric shapes like buildings, straight lines, etc.
Chromatic aberration (purple fringing) on the other hand is very easy to spot.
The other thing to worry about is lens and camera body compatibility.
I'm not sure if the Sigma/Nikon lens combo's are as much of a problem as the Sigma/Canon combo's but the problem is if you upgrade your camera body you have no guarantee that the lens will continue to work correctly on the new body.
The main issue seems to be that the auto focus stops working. If future compatibility is not a concern to you then best thing to do is take your camera body with you when you go shopping for lenses and try them out on the body to make sure they perform as expected.
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