It looks like if you go with DVDs for TOS, you’re getting either the original or remastered (i.e. enhanced effects) versions, but not both. The blu-rays, on the other hand, give you the option to watch each episode with original or enhanced effects.
Correct. However only the full season Blu-rays have both versions, the various re-releases which have only a few selected TOS episodes all have only the version with the new CGI effects.