I'm not weighing in on which is harder of the two as imo it's pretty subjective. I mained a monk last tier and moved over to ninja this tier (our ninja stopped raiding and we couldn't find an equal one to replace him) and having come from monk, ninja was insanely easy and familiar to me, all I had to do was get use to the mudra combos and weaving them.

Give me a dragon however and I'm all "AHH! This is so slow, what the he'll am I doing! Dammit I'm animation locked in an AoE" I won't even get started on my feeble attempts to play casters in savage raids

Basically my point Is, are either difficult? No. Is any job. Not really. It's down to what your use to. Faster gcds and positional? You'll have np. Switching over from a whm main that's never played melee? Maybe it'll be a learning experience but you'll get there.