@BMS
I sidelined everything for my M400 SDI pistol which represents SIG's Top Teir AR offering. It replaced my Saint Edge pistol which represents Springfield's Top Teir AR offering. I think SIG makes better guns but the Saint Victor, which is Springfield's Mid Teir offering is a better spec'd package than the base M400 or any of the special color or named M400 offerings which offer only color or handguard changes but no trigger or other appointments aside from the Ambi magazine release offered on SIG's. Now I could be wrong and SIG may have a Mid Teir but I haven't seen it. The SDI has a different barrel profile, handguard, handguard attachment method, fully AMBI lower, beveled mag well, finishing, trigger, etc...., better built and spec'd gun. Also the SIG's IMO look better unless where talking Saints with Maxim CQB or SCW rear ends. The Springfields do utilize a screw adjustment in the lower to remove upper & lower play whereas the SIG uses that rubber insert behind the trigger..... Both are very accurate IMO. TBH either would be a good choice and I wouldnt feel under gunned with the Saint or the M400 unless maybe going up against the 516G2, but that is a whole other animal, correction beast even though it's just 5.56!
The 516G2 cannot be compared to any of the above mentioned guns IMO because it is in a league of its own amongst way better built and refined guns and this isn't a DI vs. Piston thing. The base M400's and the Saint lines offer nothing in comparison to the 516G2, period IMO.
That being said, the Saint Edge line beats the SIG M400 line up until you get to the SDI's! The SIG SDI line is SIG's Top Teir DI gun and blows competition mid teir guns out of the water. The SIG M400 SDI and the Saint Edge lines can run with the big boys. The Saint Edge does have a Match Grade Comp trigger which is way better than just the fancy trigger on my Victors but the Victor got close with my addition of JP Springs lol.
All in all I would choose the Saint Edge or Victor over the M400 Tread but the SIG M400 SDI's over most ARs marketed today for just it's build quality yet reasonable cost compared to other top teir rifles. I say this speaking from a Defensive perspective not a competition or plinking standpoint. If it was for another deployment I'd run the 516G2 all day long.
I'll tell you a short story about my home protection progression since VA Lobby Day in 2020. It started with the Saint Edge Pistol with a 10.3"bbl below.
I wanted something more compact so I bought the Saint Victor PDW 5.5" bbl and transitioned to it.
I have a 516G2 that I love and a 516G2 14.5" upper on order but....
My nightstand and main HD gun is my 11.5" bbl SIG SDI "Switchblade" Pistol.
Maybe the 516G2 14.5" I have on order may dethrone it if when it ships but I highly doubt it. I love my SDI too much and can't wait to get the new M400 SDI Patrol 16" black rifle. No it's not the 516G2 but I am not back in Iraq and there is no CW2 or WW3 at the moment to really need all the juice the 516G2 brings.
To conclude my Saints are now Safe Queens next to my 516G2 while my M400SDI pulls 24/7 guard duty.
Almost forgot, once I can afford a Suppressor and .300BO Subs and Supers all I will run for HD and CQB will be Rattlers. I just can't afford to feed my Canebrake at the moment.
Anything 100-500yds out, man sized will be 516G2's/M400 SDI's and anything further will be 716's of both the piston and DI variety and eventually a Cross. SIG is on the right track IMO!
MY apologize about the redundancy. If you want a more technical comparison please let me know or PM me. Good luck. Oh and I've never had a ammo issue or malfunction with any of the Springfield's or SIG's. They will all run. Sometimes though the factory may install the wrong jet on the adjustable gas block on the Springers so maybe check the gun and break it down good before you shoot. Inspect it all! Ruger taught me this well lol! Oh while we are here also take a look at the Ruger MPR. The 18" flavor was my choice and is truly a superb gun spec'd well for an incredibly low price.