I've recently researched this on various forums.
And I wound up buying the SmartCarry.
The people who make SmartCarry (I had a very nice conversation with Carolyn) were Thunderwear sellers, but they grew concerned about their perception that things could snag on the stitching of the Thunderwear. Eventually, they broke off and marketed (and patented!) their own product. SmartCarry also uses elastic on the bands (and I understand that TW does not).
But I don't own TW, so I can't really compare the two. I'm on Day 2 of SmartCarry ownership, and I'm pretty darned impressed.
(BTW - these aren't really tuckable IWB holsters - I think they are a completely different and better concept. But you knew that.)