The spigot fitting is the superior method.
The moderator is located in line with the barrel by both the spigot and the shoulder.
With a spigot fitting the shoulder is 30 mm behind the muzzle. With a none-spigot thread it is only 15mm behind. Even ignoring the alignment due to the spigot this means the spigot fitting will be aligned at least twice as accurately as an ordinary thread.
Even if the moderator comes loose, the spigot will keep it in line. With an ordinary thread it won't.
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With an ordinary thread much of the repeatability of alignment is down to how closely the thread on the barrel and the thread in the moderator are toleranced
With a spigot fitting this is almost a non-issue. Indeed the barrel thread can be made purposely loose to ensure interchangeability and avoid seizing etc.
In practice what this means is that the spigot fitting is less likely to cause zero shift as the moderator is removed and replaced, as it should be every outing.
If you have the option, choose a spigot. M17x1 spigotted is pretty much a standard for tactical applications.
Comparison drawings of typical threads are here:
http://jacksonrifles.com/zz-silencers/files/t8-t4-manual.pdf and here
Maintenance
A quick glance should show you the important differences.
If you do choose a spigot, also get your 'smith to make a spigot diameter collar 18.5mm long. This would enable you to also fit a moderator with a none-spigot thread if you wished, keeping all your options open.
PS: With a spigot thread it will take about nine turns to screw on your mod. With an ordinary thread 15 turns. Which may seem trivial, but worth considering.