Britney's last major solo Hot 100 hit: "Hold It Against Me" (#1, Jan 2011). Xtina's last notable solo one: "Not Myself Tonight" (#23, 2010). No big solo chart hits for either since.
Overall since 1998/99: Britney leads commercially — higher album sales (100M est.), more /albums, bigger global phenomenon & cultural impact. Christina stronger on vocals, more Grammy wins & artistic reinvention (Stripped/Back to Basics).
Both icons. Britney edges pure success metrics; depends if you weigh sales or artistry. Facts over fan wars.