Which is harder, a seated dumbbell curl or a standing one
Personally, the standing. But most people will rock and use momentum, and their shoulders to pull the weight up. Perfect form will come if you stand up against a wall. This decreases the opportunity to rock ,"cheat." The seated is usually easier because the your center of gravity is closer to the weight that you are moving. Seated curls use less stabilizer muscles.
typically in a standed curl you will unintentionally use your body to curl the weight. I do seated curls.
Seated would be harder but better. Standing occupies the upper body as well and can often result in back injuries due to the fact that with very heavy weight the spine is also occupied. I would actually go for preacher chair bicep curls for biceps...it absolutely isolates the biceps.
I agree with peachtool's answer and to further explain I want to introduce you to levels of the exercise, (any exercise.)
Level 1: You and all your stabalizing body muscles are supported by a solid object (a chair) to isolate the muscle you are targeting (bicept for you.)
Level 2: You and most of your stabalizing body muscles are supported by a solid object (a stool, or just lift one leg, anything works to increase to this level.)
Level 3: You and a stabalizing body part is supported by a solid object (This may be your standing curl when standing on both feet on a firm surface.) Let me take this a bit further!