Both. Seated climbing at higher cadence is efficient and decent rear suspension helps loads with staying seated up rough climbs. But sometimes, only standing and attacking a section will do, esp on a HT. On low traction climbs, the hover/crouch is cramped and tiring but effective.
Riding a SS taught me some things about standing climbing I had not realized: It’s way less tiring if you stand taller with straighter arms; leaning the bike and levering the bars against the pedals for power, bouncing/pumping with the pedal strokes can increase traction enough to maintain grip when it’s loose. It’s knackering tho.