I also have an Aurora 9 (I love this radio!).
I almost exclusively use LiIon packs (2 cell 7.4V) with a regulator. There are a lot of new servos on the market that can handle the higher voltages of LiIon / LiPo packs and I'm slowly switching to those servos. I like the huge capacity vs weight that liIon provides. I like the ability to load test the battery (Fromeco sells a sleek little unit called an "8Ball" load checker). I can fly all weekend without charging or worrying about brown outs due to low voltage.
I don't like the A123 packs. Look at a discharge curve of an A123 pack vs a LiIon. A LiIon or NiMh/NiCad have nice gradual discharge curves letting you know your pack is loosing capacity and is in need of re-charge. An A123's voltage at take-off can be fine, but be at the end of it's capacity, having you wonder why you just crashed with a "full" battery.

If you want to stick with a 4.8 NiCad or NiMh Rx pack, you know with the Aurora, you can use a separate battery just to power the Receiver. For simplicity, I would just use a 5 cell pack.