i wonder which audio codec is better eac3 or aac in the same bitrate for stereo audio ?
I have no idea. Is there anything which can encode to EAC3?
Also which bit rates is EAC3 meant for, is it even near competitive at < 128 kbps?
EAC3 to AC3 is the same as AAC to HE-AAC.
If an original source is EAC3 it's recommended to not transcode from lossy to lossy (from EAC3 to AAC).
If an original source is lossless then go with AAC(medium and high bitrates) or HE-AAC (less than ~180-200 kbps for 5.1) for better quality.
E-AC-3 is only a slight improvement over conventional AC-3, and still carries an overhead of loudness, compression and checksum data. It requires a relatively high bitrate. AAC is more efficient and compatible on the personal computer. Maybe that is not true on h/w receivers.