Difference between seamless pipe and erw pipe

The question “Shall I use ERW or seamless pipes for my project?” is coming up over and over. Each of the two has different advantages and disadvantages, which should be pondered to take a proper decision:

SEAMLESS PIPE PROS AND CONS
Seamless pipes are manufactured out of a solid block of steel and do not have any weld seam, which may represent a weak area (subject to corrosion, erosion and, general failure)
Seamless pipes have more predictable and precise shapes, in terms of roundness and ovality, compared to welded pipes.
The main disadvantage of seamless pipes is that their cost per ton is higher than the cost of ERW pipes of the same size and grade
Delivery times may be longer, as there are fewer manufacturers of seamless pipes than welded pipes (lower entry barriers exists for welded pipes vs. seamless pipes)
Seamless pipes may have an inconsistent wall thickness across their length, indeed the general tolerance is +/- 12.5%

ERW PIPE PROS AND CONS
Welded pipes are cheaper than seamless (ERW HFI type), as they are manufactured using steel coils as feedstock in less complex manufacturing plants
Welded pipes have shorter lead times than seamless pipes, as the manufacturing base is larger
Welded pipes have a consistent wall thickness, as they are manufactured using coils (ERW) or plates (LSAW), both subject to tight tolerance control
The major “defect” attributed to welded pipes is that the presence of a weld seam constitutes a weakness factor. Whereas this may have been true in the past, this is becoming less and less true with the advances of the welding technologies in the last ten years.
Conclusion: modern ERW-HFI welded pipes are absolutely a valid alternative to seamless pipes and help end users to reduce prices and lead times between 20 and 25%.

Pipes are, with valves, the most impactful piping cost element in plant construction (as a rule of thumb, piping covers 5-7% of the total plant cost, and pipes represent circa 60 to 70% of this cost, valves 15 to 25%). These figures are average values that refer to the oil & gas industry and refer to carbon steel materials (the weight of piping may be higher for stainless steel, duplex, and nickel-alloys piping classes).