The Lrp SXX speedo will do down to a 4.5t motor and has no heat sink or fans
The Lrp SXX TC speedo will do down to a 3.0t motor and has a heat sink and fan.
Both have the same setup and software updates if you get the Version 2's
Most people would run the SXX in either 2wd or 1/12th
And the Sxx TC in 4wd or TC cars.
If you are looking for future proof buying get the TC spec and leave the fan off if running on 2wd.