Just to add a little to the post above this...
Never heard of the 4PL's receivers overheating and the latency is still faster than human perception. I use one and it has about a 50% market share of all the radios out there at the tracks in my area for the reasons of price and quality.

The telemetry could be nice on the Airtronics setup but I've never seen anyone actually use it regularly and only heard of a handful of people who do. Not worth the extra $$$.

Also rare is a glowing endorsement for the Radiopost stuff from real racers who do not also have some variation of sponsorship from the company listed in their sigs.