What features do you value?
The RX-V3900 has some nice features like Rhapsody and HD Radio. It can display short messages such as volume over HDMI (I have no idea if the 05 can do that.)
I would guess the the SC-05 has more power given that it seems to be rated with all channels driven. Ultimate demo'd the SC-05 or 07 for me, and it did not sound good. I would guess we could mostly blame that on a setup problem though.
Gene did not like that one of the SC models had frequency response issues, but that was using test signals. It's not clear whether that has any real world effect. See the Audioholics reviews.
I am not sure how MACC vs YPAO stands up. On my 3900, I think YPAO set to Natural sounds better than the other options. But the difference between THROUGH (EQ off?) and NATURAL is subtle. Maybe on some songs I might prefer THROUGH - I am not motivated to do extensive listening tests, sorry
I certainly would give the SC-05 a hard look if you want a lot of power. I cannot comment on their comparative sound. The 3900 sounds good to me.