Active race directors may have other views and opinions to add, but here is my take on it.

The AMBrc3 or Mylaps RC3 decoder has a screen which displays hits and noise levels. I see this as an advantage over the RC2 because you can see straight away if there is an issue with the loop system.
The other advantage is that the RC3 has an Ethernet port so it can be connected to a network and several PCs can receive the data.
If the decoder is connected to the internet and registered on the Mylaps web site practice data can be looked at by any of your racers.
I know of an Aussie club that has a decoder on all the time at there track so that racers can get their practice data online.
Connection between the decoder and the PC can also be done directly using a crossover Ethernet cable so no network is needed but you need to set static IP addresses in the PC and decoder for this to work.
This eliminates the need for using a USB to RS232 adaptor which is some cases can be unreliable.
If using one of these I recommend a quality adaptor such as a Belkin with the three lights on it.

Just to recap, I used to recommend that clubs with RC2 decoders take advantage of the Mylaps trade up offer to RC3 as it was a reasonable deal at around $2K to swap.
The recommendation was because of the screen, Ethernet and future proofing because the RC3 could take firmware updates but the RC2 cannot.

However, in light of the MRT and Pure RC4 transponders I am unable to make a recommendation on the swap to RC4 as the pros and cons will have different impact on each club.
So each club needs to weigh up the issues to make the best choice for their situation.