This question comes up constantly, and the answer depends on what you're trying to do. Let's run the actual numbers.
The ETB
An Elite Trainer Box contains 9 packs plus accessories (sleeves, dice, condition markers). Retail price: $49.99. That's $5.55 per pack, plus you get the accessories.
The accessories (65 card sleeves, 6 damage dice, 1 condition marker, 45 energy cards) are worth roughly $8–12 at retail. So your effective pack cost is closer to $4.44–$4.55/pack. That's good.
The problem: 9 packs is a small sample size. You're at the mercy of variance. The odds of hitting any SIR in 9 packs are roughly 10%. You'll likely hit 1–2 ex cards and possibly an Illustration Rare. That's a reasonable return.
The Booster Box
A booster box contains 36 packs. Retail: $149.99 = $4.17/pack. Slightly cheaper per pack than an ETB once you strip accessories.
Statistically, 36 packs gives you much better odds: ~34% chance of an SIR, near-certain Illustration Rare, 2–3 ex cards guaranteed. The hits are more consistent.
But 36 packs is still a small sample for chasing specific cards. You should expect maybe one SIR per 2.4 booster boxes on average.
The Verdict
Buy ETBs if: You're opening for fun, collecting the accessories, or want to give as a gift. The experience per dollar is better.
Buy booster boxes if: You're cracking for value at scale, planning to trade, or want to maximize statistical hit rate across multiple openings.
Buy singles if: You actually want a specific card. It's almost always cheaper than cracking packs to find it.
One More Thing: Secondary Market
Don't buy ETBs or boxes above retail. The math breaks the moment a box costs $200. At that price, you're paying for the possibility of a hit, not for the expected value of the hits. Unless you genuinely enjoy the opening experience, that's a bad bet.
Track retail restocks at your local Target, Walmart, and game stores. First-wave retail pricing is the only ETB math that works in your favor.