I was looking at picking up a few boxes to be primarily shot at the indoor range. Typically shot through Sig 226 9m as well as Glocks.

The brown bear/tula and quality is fine for shooting at milk jugs but at the indoor range, I am over it. Sure it goes bang but I am past that.

Really looking for something with more consistent load, accuracy and being able to shoot a tighter group.

With that being said, is a Remington/Winchester/PMC commercial box of ammo better for indoor target shooting or is the Mil-Spec type of ammo just as high quality and consistent? Or is it the opposite??

Anyone have any experience or done any comparisons?

"MIl Spec" is loaded to the procurement documents.  They may well (and often do) refer to NATO spec also.

Commercial is loaded to whatever the commercial company wants.

Nato ball in every 9mm I have tested shoots better than any commercial economy ball round I have tried. Like in many other calibers the bargain basement blasting ammo simply does not have the consistency and quality control ammo a couple bucks a box more has.
I had the chance to ransom rest some military issue m9's with NATO ball at 25 yards and the worst groups was under 2.5 inches.
With my own glocks I see about a three inch group with bargain 115 ball but a hair over two inches with NATO ball also at 25 yards.
If you are not stretching your range beyond fifty feet you are only going to see marginal improvements in accuracy

This past year I've been trying just about any brand I can find since supplies seem to have been improving on local shelves again.  I've been running 9mm ammo through a Sig P226, CZ 75 SP01, Glock 17 and Canik TP9SA.  some of the most consistent stuff I've found are the commercial offerings from military suppliers, like the CBC conglomerate that includes Magtech, Sellior & Bellot, and MEN.  MEN offers a 9mm 124gr round that is not in their military line up, but it was the most consistent, clean, and accurate stuff out of those three, and priced the same.  Swiss loaded Geco 124gr was also very accurate and clean.  Aguila 115 seemed more accurate than their 124 offering out of all four of the pistols I tried it in, but it was a bit dirtier than the above loads, but not bad.  Some batches of Federal Champion Aluminum case were very accurate, and others weren't quite as good, but decent for practice.  Battle Born Munitions ammo which seemed to be rebranded South African PMP was pretty good as well, but would give an occasional flier.

I run anything brass in all calibers.  I don't care what is it & neither do my guns.   I'd like to shoot steel case but because it sparks at my range it's not allowed.
All SD rounds are either Rangers or God Dot HP's.