Samsung released both of these SmartTags at the same time, with the SmartTag + priced $10/£10 higher than the original SmartTag to account for its extra features. These include AR Find features to help you track down your lost belongings using your phone camera, and ultra-wideband tracking for a more precise location.
If you feel that you would benefit from advanced tracking for expensive items such as musical equipment or a laptop, then definitely opt for the SmartTag Plus, otherwise the standard SmartTag should be suitable enough for tracking everyday items such as keys and a wallet.