The question is which muscle group is easier to gain biceps or triceps. Well let's examine the biceps and the triceps. First of all, just like any other body part, you're going to need to make sure that your training is correct with proper form and proper technique. Remember the Mind muscle connection is very important and the overload on the muscle. So let's just say that the training and the nutrition is on point, so which one builds easier biceps or triceps?
No matter what muscle group we decide to examine, the rate at which muscle hypertrophy or growth transpires is going to be determined by your genetic makeup. Depending on your muscle fiber type and genetic makeup within your biceps and triceps, the rate at which growth takes place is going to be determined by genetics. That's not to say that you cannot change the size or shape of the muscle, however for the most part training and nutrition aside the rate of growth as well as the shape for the most part is going to be determined by your genetics.