Balance tempo and value
Tempo helps you act now. Value gives you useful results over time. Choose the one your position needs.
Tempo: help right now
Tempo is about the board and who gets to act first. A cheap spell that removes a costly minion may let you play your own minion too. You answer a threat and build one at the same time.
Mana spent and cards drawn do not measure tempo by themselves. Look at what changed on the board.
Value: more useful results
A minion that beats several enemy minions can give good value. So can a card that makes more useful cards.
One play can give both tempo and value. But extra cards will not help if your hero dies before you can use them.
Choose what matters now
If you are close to losing, a small removal spell may be best because it keeps you alive. Returning a minion to hand may buy the turn you need.
In a slow game, using many cards on small threats may leave you with no answer later. Sometimes you can take a little damage and wait for a better clear.
Suppose the enemy board deals four damage each turn. Clearing now saves Health. Waiting may catch more minions, but it costs at least four Health and risks a buff. Check your Health and the enemy's possible damage first.
Use tempo to gain attacks, trades, or time. Use value to get cards you can play and a way to win.