It is rare these days to find a poor quality carpet. Improvements in fibre technology and construction techniques mean that the less expensive carpets wear well in the right location. Of course, some carpets are more hardwearing than others, but that isn't the whole story.
Carpets have to look good and some carpets are prone to flattening and soiling.
A heavier weight, combined with a higher density of pile, will tend to last longer than a lighter or looser pile. Combine this with a resilient and soil hiding fibre and you have the best of most worlds. Bear in mind the location that you are considering. Location is important. You will find that it pays dividends to purchase better quality carpets for heavy wear areas such as living rooms, hallways and stairs.