Either way will work, its just a matter of preference. Even the most experienced weight lifters change things up, as your body will get used to the same workout.
Most people like to group pulling exercises (back/bis) and pushing ones (chest/tris). after you are done your back workout your bi's are pre-exhausted making them much easier to finish off. Earlier this week i did my back workout and did 2 regular sets of bis until failure. Then i did 1 drop set.
For dropsets u find a weight that you can do for 20 reps lets say its 25 lbs. Right after you finish that set immediatly pick up 20lb dumbells and do another 20 reps. After that with no rest again go right to 15 lb dumbells for 20 reps. Try to get 20 for each set but you should be getting no less then 15 or the weight is too much.
25lbs- 20 reps
20lbs-20 reps
15lbs-20 reps
Note this is just an example of weight. Try the dropsets and tell me how you feel the next day