I would not sell the card at this time. There is no competitive hype around this card, so its value from a player's perspective would be on the lower end. In other words, Lugia-GX RR will be relatively higher in supply due to the amount of people opening it because of how new the set is. Demand, however, is low for the aforementioned reason.
A Lugia's a Lugia, and I would expect the value to increase over time simply because of its rarity and the Pokémon itself. I'd hold onto the card, keeping it mint and in a safe place and look to sell it when Lost Thunder is no longer considered a "newer" set. This may be a while; patience should hopefully reward you.