I haven't been to the South East, so can't compare the two, however I spent 2 days last summer going from Goteborg to Tanumshede and Varberg and really enjoyed it.
If you have your own car, then it is pretty simple to see all 3 of those in 2 days, as the E6 highway pretty much links them all. If really pushed you could see them all in a day. Without your own transport it may take a little longer as the Radio Station and Rock Carvings are a little way from the nearest station, so may require a taxi or a wait for the not so frequent buses.
Both the WHS are good, the Radio Station especially is impressive, and it is nice to see something of the town of Varberg too, you can even stay in the fort
, if you don't mind being in a hostel.
If you do have your own transport then I can really recommend staying at HavRo
. It is an organic farm just a short hop from Varberg, but with cabins that look straight out to the sea. It is exceptionally charming, very welcoming and has delicious food even by the very high standards of Swedish hotel breakfasts. (It may close for the winter though).
I guess you have your plans for Stockholm, but just to say aside from the WHS the Vasa museum is really worth a visit, as are the cool cafes and shops in Södermalm.