First, select the main control valve.( Either pressure balance or thermostatic.  I’ll list the advantages to each type.

• Thermostatic:    Anti-scald, Constant temperature delivery, Maximum flow rate is up to 3 times greater than that of the pressure balance valve, Individual volume controls (great for multiple shower heads, large shower heads or body sprays)
• Pressure Balanced:  Anti scald, constant temperature delivery.  Price

Second,

  Both valves will maintain constant water temperature (no more temperature fluctuations). Both valves have anti-scald features. BUT, the thermostatic valve has volume control and will allow much greater maximum flow. (up to 3 times greater)