Once you understand the basics of playing a melee job, you can pretty much play all of them since they are all pretty similar. Keep a high uptime by correctly using gap closers or snapshot positioning, maintain optimum buff usage, weave oGCDs between GCDs, keep your GCD rolling, etc.

MNK rotation is pretty consistent. Stuff like always Dragon Kick into Twin Snakes, use Internal Release after Bootshine and before Howling Fist and Elixir Field, Riddle of Fire pairs with Brotherhood (90s CD), etc. The variation comes from correct usage of Shoulder Tackle in fights and random Chakra procs. Adjusting proper buff usage on MNK for various fights is not too bad at all compared to other melee jobs.

NIN has some complexity from correctly managing Ninki meter (with Mug) and using different Ten-Chi-Jin moves to be optimal. For example, some fights favor opening with Fuma -> Raiton -> Suiton where as something like Exdeath (O4S) favors opening with Fuma -> Doton -> Katon. There are basic stuff like using Armor Crush correctly to avoid using during burst windows like Trick Attack.

You should try out both jobs and see what you prefer. If you like something, you will more likely spend the time to get better at it.