I was in basically the same predicament as you. I was torn between the Onkyo 605 and the SR-800 HTib (the models you are looking at are just this years versions essentially).
Ultimately, I worked it out in my head like this.
I'd like the lossless codecs, but currently I don't have a Blu-ray player and my HD-DVD player (Xbox 360 add-on) doesn't have the capability anyway. Also, when I do decide to get a Blu-ray player, if I get one with 7.1 analog outs and onboard decoding (Sony is releasing one this fall), then I'll still be able to get them with the receiver in the SR-800.
Ultimately, the SR-800 was cheaper, included speakers, and had nearly all of the capabilities of the 605. I've been extremely happy with the SR-800 (this years version - 5100 seems equivalent).
Really? "Subwoofer" isn't in the AVSForum spell checker?