I used PW and moved to AiR, keeping some of my PW units. Like most newer systems, the AiR system operates on a much higher frequency range than does PW (and thus will not talk to one another).
As a result I've found the AiR system much more reliable (never, as in ever, fails to trigger) despite distance, obstacles or orientation of the radios. AiR units do not have a 'relay' function, but you may not need it.
Profoto lamps can be triggered by a PW (or any radio) via their sync port. You can also connect (via a cable) PW and AiR units so that one can trigger the other. All AiR radios (the simple transceiver and full remote power control transceiver) save the TTL-N/C transmitters (which are transmitters only) have input and output jacks.
You can control the power of a Profoto AiR lamp with an AiR remote while triggering it with a PW or any other unit at same time (via its sync port).