Pans are usually more convenient for travel and the usual choice for portable paint boxes. However, with pans, it usually takes longer for the paint to soften with water so some painters spray water over their palette a few minutes before they begin painting to soften them up.
Some pan brands are softer than others, those containing honey are often semi-moist in the pans and the colour lifts easily. Using pans can cause more wear of watercolour brushes if you scrub the pan to pick the colour up.
Tubes contain liquid paint which is priced lower per ml than in pans. Some painters buy tubes and squeeze them into empty pans but it should be remembered that as the water evaporates the paint shrinks in the pan.
The formula for the paint in the pan and tubes is different for some brands - for example Winsor & Newton who extrude the paint for the pans and place it in the pan as a dried cube. They say that their two different formulas mean that their pans are easier to re-wet than paint from their tubes after it has dried on the palette. For other brands the paint is the same for both - for example, Schmincke fill their pans with the same paint as their tubes and this requires three cycles of filling and drying for a pan to be full.