Just beat 10 today. I think I would call it easier than 9 for the most part, considering that I wasn't scrambling to learn special weapons against castle bosses as much, and the "blindside deaths" of 9 have been more or less eliminated (awesome, although the Castle gets a little aggravating in its later segments, but nothing too serious). The fortress bosses kick butt, for sure.
A lot improved in this game. The level design is superior, you're no longer paying for difficulty levels as DLC, there's no ridiculously expensive cosmetic item to distract you, tank pickups are more abundant but in tricky locations, the common reused tunes (RM intro and stage clear) have been remixed, Get Weapon screen includes character art, and only 12 "Achievements" remain with the bulk of the "Challenge" mode being actual separate tasks to complete (including BOSS REMATCHES that you can jump into at any time). Awesome, awesome, and more awesome.
One or two missteps are still around, but they're all pretty minor compared to my gripes with 9. However, using B for weapon switching on the Wii Remote (see last page discussion) is DEFINITELY the most serious issue with me. It's way too easy to tap B during a white-knuckle jump. Control options do not allow you to change this (only to swap fire and jump; WHY anyone would want to do that is beyond me). Next time they do weapon switch, they need to either make it A and Minus, or just put full mapping in the Options and leave it at that.
A shame that I occassionally wind up with the wrong weapon in what is otherwise a title that definitely surpassed my expectations (9 had me skeptical, and the nostalgia of the NES style wears thin when MM3 is already your most frequently played NES game). Very pleasantly surprised considering I haven't gotten my hands on the DLC yet. With Bass, another Endless Attack, and three Special Stages on the way, I'm definitely excited.