What type of outdoor extension cord should I get for heavy use?

Get STW instead of SJTW. The “J” in SJTW means junior—thinner insulation that’s fine for light indoor use but not built for weather, mud, or being stepped on. STW has a thicker jacket, better abrasion resistance, and is less likely to crack in the cold. The price difference is usually a few bucks, not worth saving if the cord splits after one winter.