You use the 75°C rating when both ends are rated for 75°C
In order to be able to use a THHN wire at its 75°C rating, all the splices and termination-points in that stretch of wire need to be rated for at least 75°C. This is true of some styles of splicing devices (such as Polaris-type insulated mechanical connectors) and of breaker and panel lugs, but most notably not of wiring devices such as receptacles and switches.
As a practical result, this means that when working in conduit, feeder wires, along with wires to hardwired appliances, get to use the 75°C column, while general lighting and receptacle circuits are restricted to 60°C ampacities. Of course, this only applies to wires 8AWG and larger, as NEC 240.4(D) limits the breakers for 14-10AWG wires to their conventional (60°C) ratings anyway.
The 90°C column, by the way, is only conventionally used as part of applying derating factors for ambient temperature, or more commonly, conductor counts in a single conduit. While it's possible to run wire at that temperature by using a pigtail of fatter wire to interface the 75°C lug on a breaker or panel to the 90°C conductor via a 90°C rated splice, this is uncommon, and would only generally be done if conduit fill is a severely limiting factor or if existing conductors cannot be replaced, but can be safely run at the higher temperature rating.
Also note that there is no such thing as a heat sink for a breaker -- the calibration of a breaker's thermal trip depends critically on the temperature rise environment the breaker is in, and taking it outside the UL 489 testing envelope would throw that calibration off, heatsink or no heatsink.