Thermocouples- inexpensive and can operate over a wide temperature range. A thermocouple is created whenever two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. This thermoelectric voltage is known as the Seebeck voltage, and this is what is sent back to your PID.
RTDs- Resistance temperature detectors are simalar to thermocouples but use 3-wires instead of 2. They operate on the principle of changes in the electrical resistance of pure metals and are characterized by a linear positive change in resistance with temperature, hence the 3rd wire. RTDs are slightly more accurate but if you calibrate your PIDs you will be fine with thermocouples.
I use thermocouples in my HLT, and BK, and use a RTD in my MLT.
If you are running your control panel at 30 max amps and using only two elements, then I would suggest 3 contactors. 1 to turn your main power on using a 120v switch, and 2 for your elements(1 for the HLT, and 1 for the BK). But remember you are going to want to run ONLY 1 element at a time. A 3 position selector switch will allow you to do this. 5500w=23amps@240v, this will leave you enough power to run your pumps safely.