The 5.8 is a beautiful boat and the basic rule I've found is the newer they are the lighter they are. All up ready to sail the 84 model probably weighs around 200kg, the 87 around 180kg, a new one weighs around 160kg. I'd lean towards the newer one but condition of sails, tramp etc would decide the job. Have they both got the NA bow foil and big jib? Have they both got original masts as the newer masts are a fair bit lighter and easier to raise so if one was replaced in the last few years it would have an advantage
Good luck with your purchase and post pics