As usual I suggest getting the 671 for the step up in power. And you get network audio with Pandora as icing on the cake. If you don't care about network audio, scoop up an RX-V667 while they last which has pre outs, which the 671 lacks.
But your statement really interests me. I can get leftover 667 for about the same as 571. I don't need internet radio. Is there nothing else I give up? One of the key features for me is power off forwarding of hdmi - My wife watches alot of TV and likes to use just TV speakers - this is much easier if whe does not have to switch TV inputs, or, mess with the receiver. Deos the 667 do that? I'd like to have more power than 571, pre-outs (both just in case).
edit: darn, it appears the 667 does not have front panel usb input (for ipods etc)