Dolby True HD does not have a "Core" of DD, only DTS HD has a "core" of DTS. I believe that the UP5000 (and possibly the LG) WILL reencode the Lossless formats to DTS @ 1.5 mbs. Which should be better than regular DD.
Really the re encode only effects the Dolby THD and DD+ soundtracks, because DTS HD (both HR and MA), have the DTS core all ready. So as long as your receiver can decode DTS, that would be your best solution.
I think most movies that only list a Dolby THD track, still have a "hidden" DD track, but it is not part of the THD track. Which in any case would be better to re encode the THD track to DTS.