From the academy, the one that costs token to sign is much stronger (faster trainer) than the one that costs nothing. The fast trainer people are talking about is that 4* sold by the bots or by the whales, as a 4* spawn is so good, better one cost even more tokens to sign, most players don't sell them but use that directly instead. Whales, usually run out of in-game cash but have limitless tokens, can sell token-signed young for cash which gives better exchange rate than investments (as a 4* is too weak for their tastes).

They grow up in value extremely fast when trained and allowed to play in matchs regularly, and usually perform very good regardless of quality (a lower quality team result in lower possession however).