Use Topt, it gives the most accurate measurement. For domestic sized rooms a sensible target would be around 0.3s. Broadly speaking a HT setup is better being on the dead side (lower RT60) and a music setup a little more lively (higher RT60). Ignore RT60 at low frequencies and use the frequency response and waterfall displays to see how the room is behaving. For more pointers on this best chat to the folk in the
Acoustics forum.