The difference between the Helios and Eurus is that the Eurus internals are 100% digital. The Helios does some analog to digital conversions. That relates to faster and more precise response times.

As for the 407, 408, and 409 receivers. The 407 and 409 are newer and work with the newest KO module for the fastest response times. The 409 runs a coax antenna and has better range than the 407. I used both the 409 and 407 and haven't seen any difference in range.

I don't know if the 408 will work with the new systems, but I do know it's a slower frame rate.