If you can cycle, hire bikes at either Assen or Emmen and cycle, its very very flat (despite what the dutch think!). You could cycle to borger and back in a day, as a guide. Otherwise I'd guess both have buses. Grongingen is further away, however, it might quite groovy to visit (John Peel has a favourite record shop there I think). The northern most Hunebedden are south of there, and less visited. In my opinion visiting the borger hunebed is on a par with a visit to stonehenge - a bit of a let down. Museum is nice though.