Here is my opinion on the costs and benefits of the options.
1) HAI omnistats. The simple ones are inexpensive and work pretty well, but the build quality can feel a bit cheap. The more advanced ones have lots of additional features that can work with ELK, but are easier to integrate with HAI. No additional hardware needed to connect them, just hook it up to your normal thermostat wiring and run 4 wires(preferably twisted pair like cat5) to an M1XSP. Sometimes requires a little bit of electrical savvy to connect, but the instructions are very clear. Future WiFi option is quite appealing if you don't already have a data wire.
2) RCS. The thermostats connect to a control unit via normal thermostat wiring(4 wire), utilizing your existing thermostat wiring. If you are going to use zoned heating this can be one of the cheapest setups as each thermostat is only a dumb wall display unit, the brains are in the control unit, and this reduces the cost of the thermostats. The control units connect to your HVAC equipment and can be placed near your furnace or zone control panel(presumably where the existing thermostat wires go) You only need 1 data wire from the M1XSP to the location of the zone control units, no need to run data wires to each thermostat. Great retrofit option.
3) Aprilaire 8870. Never heard a complaint about the products, very good reputation. However, additional hardware is needed(protocol adapter and distribution panel) to connect to the M1XSP, raising the cost beyond other options. Thermostats are expensive, but can control almost any furnace(2 stage heating and cooling, heat pump or conventional, lots of control adjustments, etc). Great option if you want reliability and compatibility with almost every furnace, but a tad expensive if you have a simple furnace or only need 1 thermostat. Also does not have an easy way to display outdoor temperature except via the message line.
EDIT: Forgot ELK does not support RCS zone control panel, just the control units. Updated info.