The SR-D-IT and TAPS are both "dumb" switches, in and that they are one button per device, and will only active the "native" functions on the device, which in the case of a Surefire light is momentary only, and IIRC, for Steiner devices, single tap momentary, double tap constant (I am not sure off the top of my head whether or not this applies to the OTAL).
The TAPS PRO is programmable, and can be programmed for various different constant functions with the light, and you can also program one button to active both devices.
The TAPS PRO is expensive, and both TAPS models are large in terms of form-factor. The Surefire SR-D-IT is not cheap or small, by any means, but it can be cut down (removing the rail grabbing function) to fit in non-railed positions. The TAPS are both M-LOK/KeyMod compatible without an additional rail section (and with some creativity can be direct mounted to other modular forearms), while the SR-D-IT you're pretty much restricted to using a rail section, or Velcro/zip ties to get it into position.
Some users have reported the Insight-pattern plug on the SR-D-IT slipping out of Steiner devices due to design issues, requiring simple, but aggravating modifications to the plug, this is supposedly not an issue with the TAPS, though I have not first-hand used the TAPS with a Steiner/LDI device.
~Augee