TLR-9 (flex) is a winner for the NHC Double Stacks.
TLR-9 is a flush fit with the end of the slide. What's also as good is with its inline batteries its pretty thin keeping true to the 1911/2011 slide. Its not fat like the X300 or TLR-1 models with their side by side batteries.
TLR-9 also has great lines in profile which add to the look of those Sandhawks, TRS Comps, and BDS9s, I was never a fan of the X300 or TLR-1 ups and downs.
TLR-9 with its inline batteries loads from the front so you don't need to remove the unit to change batteries.
I prefer the TRL-9 switches (like the TLR-7) to the X300 or TLR-1 so that's a bonus.
With its 1k Lumen,10k candela, 200m distance its no slouch for a pistol mounted light.