The Scalper Tomahawk is nearly identical to the Survival Tomahawk, except that it has one less point of concealment (26 vs 25) and has slightly less range (225 vs 200). It is highly recommended that players use the Survival Tomahawk over the Scalper Tomahawk once the former is unlocked.
The name "Scalper" is a reference to a common stereotype regarding American Indians; namely that they would use tomahawks to "scalp", or cut off the hair and head skin (a.k.a. "scalp") of their defeated enemies and claim it as personal trophy. This is also referenced in the weapon's description.