These are two entry level receivers manufactured by Denon. Denon has been in the audio industry since 1947. Developing hi-fidelity audio equipment. Both are an excellent value so if you're considering one or the other I thought I would break it down for you. A detailed review of the DENON AVR-591 is here
3D compatibility is included despite the low price tag on this product . Pass-through makes it capable of passing a 3D video signal from a 3D source to a 3D HDTV. Regardless if you need or want it, it's a good "future-proofing" feature to have if you ever change your mind.
Audio return channel (ARC) is one of the new features supported by HDMI 1.4, but just because a receiver has HDMI 1.4 doesn't mean it has audio return channel. That's because it's not a mandatory part of the HDMI 1.4 spec, and manufacturers can choose whether to include it.
Confusion aside, it's a worthwhile feature that allows TVs to send audio back to an AV receiver over the same HDMI cable that is carrying video from the receiver to the HDTV. That's useful if you're using your HDTV's over-the-air tuner or perhaps one of its built-in streaming media services (such as Netflix or Amazon VOD), and want to listen with your surround sound system. It's not a revolutionary feature--it's only saving you the hassle of making an extra connection from the TV's digital audio output--and it only works with new HDTVs that support ARC, but it's still a nice bonus to simplify your home theater setup.
Dolby ProLogic IIz
ProLogic IIz made its debut last year and it's become widely adopted this year. This feature comes on many
of the midrange AV receivers being manufactured today. Heres a demo of the technology.
iPod/iPhone via USB
Makes it easier than buying a dock!