As others have said, I would definitely go for Mega Gengar, assuming you don't have another mega you would rather use. It's a great mega, and should be a good addition to your team.
In general, I find that items like Spell Tag and other 1.2x boosters are useful in game, but not that good competitively. It's not worth just boosting one of your moves (e.g., your ghost-type move) a bit when you can use something like a choice item or life orb and get a bigger boost across the board (especially since competitive battling involves so much more switching Pokemon than in-game battling does, anyway). If this is for in-game, it doesn't really matter, though.