Personally I thought ZX was harder than Advent.
Off topic: I personally didn't like ZX, the main gameplay felt too much like the Zero series (Model ZX plays the exact same too). The overworld was just too annoying to navigate (anyone remember getting the Nurse Missions to get a Sub Tank?). And I always found the Z-Saber to be weaker here than in other games for whatever reason.
In comparison, Advent felt much more like a breath of fresh air IMO given that you're playing a an entirely different MM than either X or Zero, that both main characters played differently (Ashe for the win!
), the revamp to weapon energy, an easier to navigate world map with lots of warp points, and that the various transformations were utilized better for puzzles/gameplay. Also dat "Model a" minigame was all kinds of awesome.
My only complaints are the story (which is tough to follow and ends on a bigger cliffhanger than Legends 2), and the less connected world map.
I'd put Powered Up! in the same boat as 7/8/Bass due to the changes in physics and that you can't shoot through walls. Good for beginners, but not if you're looking to majorly get into the series.