Hot-aisle containment is generally preferred for high-density environments because it efficiently captures and directs hot exhaust air back to the cooling system, increasing temperature differentials, and improving cooling efficiency. Hot-aisle containment should be considered early in the building design, as it may require a double ceiling. Without this, it's very hard to create a vertical hot aisle containment. 

Cold-aisle containment is typically easier to implement, particularly in retrofit environments, and may be preferable where building constraints limit ceiling height or return air pathways. 

Ultimately, the optimal approach depends on the complete data center design, including cooling architecture, structural constraints, and operational requirements.