Chrono-chaos (bigfootm?) Is absolutely correct. It does depend on gear and skill, to a point (by this I mean that no matter your skill, flourish + JK = fail). Let's say that you have an avenger. If you have this, yet at doing Roarmulus for the first time, it will not be easy at all. If you know what you are doing and how to kill them, however, it should he very easy (ex: I beat roarmy in 5 mins with shockburst brandish as only weapon- awesome sword)
On the other hand, with jelly king, you might have a difficult time with no piercing defense, but you sill can manage to kill him since there is a "all out brawl" strategy, which most people are used to. (arena, Decon zones, etc.)
Based on this, I would have to say roarmy is harder. Damage, projectiles; and the strategy needed might just be too much for inexperienced starters.

Basically, JK> roarmy.
I suggest you do JK more often anyways, because the crown payout is nice, and it's token rewards are some of the best weapons in the game.
The only reason to do Roarmulus is to get tokens to buy the guns or bomb or armor, or if you are helping people on a journey. Other than that, roarmy sucks.