Well, take for example the BR of Nemesis compared to the dvd. I can tell that it's nicer then the dvd. My BR of it was part of the Stardate collection set, so that's why I have it. However, I would not have upgraded to BR from dvd (for that movie specifically) if I was buying the BR seperatly. If the same quality upgrade can be said for the dvds/BR's of Enterprise, I can settle for the dvds in this case.
Based on that post, Mage
, I would have to say your choice is DVD. The comparative increase in quality for ENT on Blu-ray is much smaller than the Nemesis movie release.
However, if you don't own the series, it's worth hunting around for "deals" as I suspect there isn't a huge discrepancy in pricing between the two, and for the sake of a few pounds/dollars whatever, I would still advise going for the best product available.