StatTrak™ adds a kill counter to a skin and is a separate variant with its own price, usually carrying a premium over the normal version.
The size of that premium depends on demand: popular weapons like the AK-47 and AWP hold a meaningful StatTrak markup, while less-used weapons may see little difference.
If you mainly value the look, a normal variant is the cheaper buy; if you want the counter and better resale on a popular weapon, StatTrak can be worth it. Always compare the StatTrak and normal listings separately — they are distinct SKUs.