Hi Hypra ... To answer your question, it's actually good to do both ... The variety will keep you from reaching an exercise plateau. One day you could do difficult resistance the next easy OR one week resistance the next easy... It's all good ....
Depends on what you're gonig for - muscle building or endurance. If you go really fast and get your heart rate up, then you're building endurance, but you're not challenging the muscle.
If you push really hard, you're still going to get your heart rate up but you're really going to work the muscle and build muscle and strength.