Light or Heavy

BOTH.  You should lift BOTH.

Light Weights, light enough that you can do anywhere from about 3 sets of 12-20 reps before fatigue sets in.

Heavy Weights, heavy enough that you can only get in about 5 sets of 6-10 reps and no more.

Using lighter weights ( -and I am not talking light as in 3-5 lb dumbbells though you guys!!) with more reps is just as effective at muscle building with the heavy weights.  As long as you train to failure A.K.A. fatigue OR not being able to finish your last rep in a set with proper form.  Your muscles react the same way upon REACHING FATIGUE regardless if it's your 6th rep with heavy weight or your 25th rep with light weight.  Just enjoy the benefits of using both styles.  It's just fun to mix things up.

Combining the two lifting styles can give you great results.  Say you do shoulders with light weight for one workout.  Next time you hit shoulders go heavier and shorten the reps.  You can even mix the two styles up in a single session.

Always ALWAYS continually challenge your muscles in ways that forces them to adapt.  Keep the muscles guessing.  Be in control.  Don't fall into the routine of doing the same workout, made up of the same exact exercises using the same amount of weight over and over again.  

HAPPY FRIDAY!