Sectional gutters are assembled from shorter pieces joined together along the run. Seamless gutters are formed in continuous lengths for each straight roof section.
Main tradeoff
Fewer field seams generally means fewer joint locations to maintain, which is a major reason seamless systems are popular for professional installations. Sectional gutters can be easier for small DIY repairs or additions because individual pieces are readily handled.
Neither format fixes a bad drainage design. Pitch, outlets, downspouts, support, and discharge still determine how the system performs.
If the issue is already affecting water flow or the gutter is physically damaged, see our gutter installation page.