Jeff,
I am having a similar dilemma, where I wish to have the best of both worlds. Given your advice would the BDA & SP3 provide the opportunity for the ultimate (?) 2 channel and surround sound. Or, is it just better to go for the BP26 and SP3 ???
I have not heard the analogue passthru on the SP3, but assuming it is as good as the SP1.7 and SP2 (a safe assumption right James) then the BDA/SP3 combination would be hard to beat.  
My personal ultimate would be 2 separate systems, one for 2 channel BP26/BDA and one for multichannel SP3, this way you can optomize speaker selection and position for both.  
Unlike James I do not have the space available for 2 (or 3) separate systems without moving some of the wife's stuff  

 so I have had an SP1.7 in a 5.1 system for over 6 years.  I am very happy with this unit, the DD and DTS processing is hard to beat, and have not found anything worth replacing it with yet.  
With the SP3 I hope to get the same sound quality plus the new lossless codecs.  I am thinking of teaming it with the new OPPO 93 (plays movies and Flac files from a USB drive as well as any shiny disc) to get excellent surround and good 2 channel audio.  
Jeff