For 720p, a BD50 would be a waste of space, resulting only in an unnecessarily huge download. As I understand it, with highly efficient video codecs like h264 you eventually get to the point of diminishing returns as the bitrate goes higher, yielding ever-larger file sizes with minimal visible improvement.
Along a similar vein, I could certainly put out my audio tracks in 24-bit resolution if I really felt like it: after all, Pro Tools does calculate all its results in 32-bit floating point for the sake of accuracy. However, since I'm dealing with laserdisc sources that were 16-bit from the start, there isn't actually any more detail in them than this to begin with. As long dither is properly applied, any quality difference when converting them back will be quite miniscule. And since lossless compression schemes seem to work more efficiently on 16-bit material anyway, the choice is clear.
Now, if there were true 24-bit copies of the soundtracks to begin with, or if the video sources were sharp enough to justify 1080p, then releasing them at these quality levels would be worthwhile. But given the sources that are actually available, playing the numbers game isn't going to create any additional benefit.
Depending how many tracks are included and how much disc space is available, using redundant copies in Dolby and DTS together may be impractical. I've done a bit of testing and have found that TrueHD gives a significantly smaller file size on its own, since it contains lossless audio only, where as DTS-HD MA includes a lossy core as part of the same file; but since the Bluray format requires TrueHD to have an accompanying AC3 track as well, the total ends up being about the same. That is to say, the total file size of TrueHD + AC3 ends up being approximately equal to DTS core + extension, depending what lossy bitrates are used.
Therefore, using both DTS-HD MA and AC3 together would require significantly more disc space. Whether this would be a problem or not remains to be seen.
Since not as many people are able to hear DTS properly, due to their equipment or software not fully supporting it, my general inclination is to say that Dolby is the way to go. The only thing that could potentially be an issue is that not all Bluray authoring programs support TrueHD in the same way. It isn't a problem with the audio itself, but rather with how the authoring software handles it: the TrueHD and AC3 streams are supposed to be interleaved with each other for compatibility with the Bluray format, but the Dolby encoder creates them as separate files, so results may vary. It looks like tsMuxer used to have a problem with this, but it may have been fixed.
I will continue to investigate these matters, and report back on my findings.