I believe Agile is type of Iterative SDLC while spiral is type of Incremental SDLC. Scrum is one the type of Agile other are DSDM/FDD/XP etc.
All SDLC after waterfall followed same set of acts(Requirement Analysis, Design, Coding and Testing) in some different combinations. So basic set of action in sequential OR Iterative OR Incremental are same.
As far as Agile and Spiral are concern both have common advantage
1.Changing Requirement handling
2.Short term releases
3.Risk management is easy due to shorter duration of SDLC
4.Cross team helps product and project going smooth
