The obvious answer is ‘it depends’. As to remasters, some are a disaster (i.e. The last set of New Order reissues) but many are brilliant- that’s particularly the case when records that were mixed for vinyl originally are properly remastered for CD/digital. Sometimes this can make the records come alive again - two recent examples of records I have had for years sounding completely new and fresh and wonderful on remasters are the recent versions of The Associates albums (especially The Affectionate Punch) and the fantastic Big Star box set Keep An Eye on The Sky.
I would always prefer to have the ‘extra tracks’ on a separate disc so the original album stays as intended. I normally don’t find them much more than a curiosity and don’t often listen to them that much. There are some wonderful exceptions- Keep an Eye on the Sky is an example where the demos, alternative mixes etc are often a revelation. My favourite ever is the reissue of Prefab Sprout’s Steve McQueen which has an extra disc where Paddy McAloon records new solo versions of almost the whole album, his voice wonderfully altered by 20 odd years of hard living. The album suddenly has a whole new dimension to it.