Obviously, if price is not a factor then it is a no-brainer to go with the 605. I struggled with this decision a few months ago and finally went with a refurbished 505 direct from Onkyo because the difference between that price and what I would have paid for a 605 was just too great.
But you really do have to look at the 505 as if it didn't have HDMI because all the HDMI connections are good for is switching between HDMI sources. As others have pointed out, this means you won't be able to use the highest quality audio formats. I'm not an audiophile, so for me the DTS/Dolby Digital surround is sufficient. It also means more cables, since you need separate connections for audio and video. For instance, I just ordered an Oppo 980 DVD player which can play DVD-Audio discs, but to get the full effect of that I would need to hook up the individual 5.1 analogs (7.1 if I had the rear surrounds) rather than sending a signal over HDMI.
A last but very important point is that the 505 does not switch/upscale video. For your scenario, this means you could run the HD-A3 and the XBOX to the receiver and then go HDMI from the receiver to Sammy (with separate audio connections for each), but for the DVR you might as well run the component directly to the Sammy because the 505 won't do anything for you and you'll need to switch inputs on the Sammy when going between the DVR and the other sources. If you have a programmable remote (e.g., Harmony) then you can alleviate the hassle somewhat. The 605 has an "HDMI monitor" setting that will output any video input over HDMI, performing upscaling if necessary, so you would be able to hook everything to it, leave the Sammy on HDMI input, and switch sources with a touch of a button on the receiver.
Note: Apparently the 605 HDMI monitor feature scales everything to 720p, so if you have a 1080i component input then it will be output to your 1080p Sammy after conversion to 720p, which the Sammy will then have to upscale back to 1080p rather than just deinterlacing the original signal.