a) You can reduce your drag by slipping through the water more easily. You'd achieve this through a better body position and better streamlining.
b) You can increase your propulsion for the same level of physical effort. You'd achieve this by developing your catch, pull, rhythm and timing.
Both of these would increase your overall swimming efficiency. Increased efficiency means you go faster for the same effort, or the same speed for less effort.
So which is more important? What should be my priority?
That depends on you. If your drag is very high working on your propulsion will only bring you small gains in speed. Instead, working on improved streamlining and body position will boost your efficiency and so your speed.
Alternatively, if you have been swimming for a good while and have been told you have a nice stroke but are still slow, a focus on propulsion makes sense. If you are in this situation we know how frustrating it is to be told you are doing everything right but you aren't making any speed improvements. This is more disconcerting than for someone who has a clear problem to fix! Start focusing on the right thing and you too can make big strides forward.
If you have access to a coach, ask them to watch you swim and tell you whether drag or propulsion is a priority in your stroke. If you don't have a coach, next week on Feel For The Water we'll be posting some techniques you can use to determine which area is a priority for your individual stroke. It's one of the most useful facts to understand about your swimming.