Jim, the Explorer was the second "Voyager" built out of 5. The first 3 are practically identical and the Navigator and Mariner had a few minor changes. There is no Diamond Lounge on the explorer but they have been serving complementary wine, champagne, and soda to Diamond Members in the Maharajah's Lounge on deck 5 every evening from 4:30 pm until 8:00 pm. Diamond Plus and Pinnacle member could go there also but they could have other complementary cocktail.
I personally wouldn't pay the extra $ to bump up to a GS. You can buy a lot of goodies for the difference in price. Some people may think having a GS is worth it for the perks of the Gold Card.