A key difference between Blackjack and Spanish 21 is that all 10s are removed from the deck in Spanish 21. A Blackjack occurs when the first two cards that are dealt equal 21 a combination of a face card (Jack, Queen, or King) and an Ace (which can have a value of 1 or 11, depending on your hand).
In blackjack, the cards that are better for the player are the high cards, specifically the 10, J, Q, K, and A. This is because these cards increase the probability of getting a blackjack (an Ace and a 10-value card), which pays out at a higher rate.