Start with the service condition: pressure, temperature, and the fluid being carried. High-pressure and high-temperature service generally demands seamless pipe, which has no longitudinal weld seam to fail.
For lower-pressure structural or conveying applications, welded pipe offers better economy and full availability in long lengths. The weld itself is today produced with processes reliable enough for most commercial duties.
Match the material grade to the environment: carbon steel for general service, alloy steel for elevated temperature, and stainless where corrosion resistance dominates.