Here's a question: In a deck that only (or for the sake of discussion, primarily) uses EXs, would it be better to tech in 2-2 Garbardor (negates the near autoloss against Sigilyph and is helpful against eel/altaria/whatever else) or 2 Terrakion to just kill the sigilyph that stop you in your tracks? Both have advantages: Terrakion is helpful as backup and requires few energies to attack while Garbardor takes advantage of multiple other ability decks without getting in the way, but both have flaws: Terrakion can ruin the energy distribution and can mess with the whole tanking thing, while Garbardor is a stage 1 that needs a tool and can't fight to save its life. I personally am thinking of tri or quad Terrakion-EX, but tried to phrase the question broadly so the discussion isn't too limited.
ahh yes, now this is a tough question to ponder.
As you probably already know, I am a bit biased towards the garbodor. But alas, Terrakions sure are spiffy - even in the BW on format where they are arguably a bit less necessary.
It's really a cost-benefit analysis trainer. I would advise sitting down and testing as many projected top tier decks as you can, with both variants, and construct a spreadsheet with your results.