Again according to MIL-HDBK-5J

"2.6.7 15-5PH
2.6.7.0 Comments and Properties — 15-5PH is a precipitation-hardening, martensitic stainless
steel used for parts requiring corrosion resistance and high strength at temperatures up to 600EF. Alloy 15-5PH
has good transverse ductility and strength in large section sizes. This material is supplied in either the annealed
or overaged condition and is heat treated after fabrication. Parts should never be used in Condition A. When
good fracture toughness or impact properties are required, both at or below room temperature, conditions H900
and H925 should not be used. Conditions H1025, H1075, H1100, and H1150 provide lower transition temperatures
and more useful levels of fracture toughness than the H900 and H925 conditions. The H1150M condition
has the best notch toughness and is recommended for cryogenic applications.
Manufacturing Considerations — 15-5PH is readily forged and welded. Forging procedures are
similar to those used for 17-4PH, the forgeability of 15-5PH being superior to that of 17-4PH in critical types
of upset-forging and hot-flattening operations."

The big difference between 17-4 and 15-5 is that 17-4 is an air melt grade, while 15-5 is vacuum arc or electroslag remelted.  (Yes, I know you can get 17-4 vac melted, but that’s a special, not a commodity.)  The vacuum or electroslag remelting does help some in making the steel cleaner, no matter your definition of clean- but what it REALLY does is change the solidification pattern.  Air melt ingots freeze from the sides inward, so that the inclusions and slag go to the middle and the top.  Remelt material freezes from the bottom up, so the bad stuff goes to the top, not the middle.

The end result?  17-4 can have pipe, while 15-5 doesn’t.

A lot of 17-4 these days is made by continuous casting: and I’m sufficiently ignorant of this process to be unsure if the above generalization holds.  But for the hydraulic parts my company makes, when we want to make something that we don’t want to leak through the center, we will always use 15-5.

I am working with sheet (coil) product, so keep that in mind.  I can find most of the PH grades in either AOD or remelted variations.  This is a critical factor if you are dealing with large section material.  The reduction in segregation and nonmetallic impurities with ESR is significant.  In light gage cold rolled sheet it doesn't matter much.  The heavy reductions and multiple process anneals tend to do very well at breaking up undesirable microstructures.
For heavy plate concast may be better than old ingot material, but it may not be good enough.

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