It's not even close to as clear cut as that. More like this:
Heavy weight - low reps = strength
Light weight - high reps = endurance
Like tzan said...it's not exactly your strength training routine that determines whether you will add or lose weight, it's more diet and cardio (again, not as clear cut as that...as nothing ever is).
One more point, you really shouldn't do a 5x5 routine strictly for arms. For example, you shouldn't really be lifting a weight that you can only do for five reps on barbell curls...it places too much stress on the elbow joint. You shouldn't really go below 7-8 reps in assistance exercises.
a muscle doesn't give a crap how many reps you do, or sets; it doesn't have a counter you know. What do you think is the difference between 2 sets of 10 and a 20-rep rest-pause set? Ans. There is none. The muscle only knows a harder workload than its seen before, the mechanotransducers start a chemical cascade to make the muscle increase its cross-sectional area (CSA) so that doesn't happen again . . . Admittedly, it tends to be easier to overload a muscle in a shorter time with heavier weight. But that is not a law. Serge used to do 10sets of 20rep bench as his chest workout, and my avatar proves that builds mass . . . There are a hundred other pro- examples. There are also guys who work real heavy 1 set each exercise: Mentzner, Yates . . . That old "law" is a completely, scientifically disproven old myth. However, probably 90+% of athletes at our level on this board, were we to use light weight 20-rep sets, would probably tire aerobically before our muscles got a significant work overload to induce growth. So for me -- heavy is better. And you too I guess. But its not a law just our experience/temperment/metabolism, some only grow with over 12 rep sets. Go figure.