In a former life, I used to service the Fire and Gas installation where I worked…
So to answer your questions…I offer the following:
Rate-of-Rise (ROR) heat detectors operate on a rapid rise in element temperature, irrespective of the starting temperature and can operate at a lower temperature fire condition than would be possible if the threshold detection point was fixed.
A fixed temperature detector operates when the heat sensitive element reaches the point of changing state from a solid to a liquid. there is a delay due to thermal lag in the accumulation of heat at the sensing element so that a fixed-temperature device will reach its operating temperature sometime after the surrounding air temperature is a lot higher. Potentially there may already be a fire!
Smoke detectors come in three flavours…optical, ionization and a combination of both.
Most installations for domestic use, use the optical detector, in industrial applications the ionization type requires a licence because of the radioactive element contained within.
If you do go for smoke and heat, dont forget to have a testing regime in place…..We tested our systems every three months…