Hello,
Though I'm not certain, I would think the Mobile Intel Celeron M 420 performs [marginally] better since it has 1MB L2 cache compared to the Intel Atom N270's 512KB L2 cache.
Do you have a laptop in mind? If so, what is your budget? A Mobile Intel Celeron processor equipped laptop is likely going to have low end integrated graphics, and together, they probably won't give you good performance for graphics design tasks. However, surfing the net, word processing, listening to music etc. would be okay.