I'm in both my school's symphonic and marching bands. Let me tell you, marching band is at least four times as difficult as concert band. Between lengthy rehearsals, band camp, football games, competitions, and parades, marching band requires far more time and effort. With concert band, you can get away with not knowing your part. In marching band, you need to know your music perfectly. And if you miss rehearsal in marching band, you're holding the whole band back. Missing a concert band rehearsal rarely affects the entire group, unless you're a soloist or first chair I guess.
In conclusion, marching band is much harder than concert band.
If they were to play music of the same difficulty, then yes, marching band is harder. But if concert band music is significantly more difficult, then it could be about the same level of hardness. Heh heh. I said "hard". I have the humor of Bevis and Butthead. I am ashamed.
Marching band is much harder this concert for many reasons. Firstly for every concert band performance, you are permitted to have your music in front of you. In marching band you must have you music memorized. Also, concert band requires very little physical output. Marching band, in my experience, is a four day a week deal with many hours each day. I speak as a member of both groups and in my opinion marching band is much harder than concert band.
You still have to sound as good as a concert band, with the addition of marching. Playing well while marching is extremely difficult, and your mouthpiece will bounce against your face and ruin your tone if not done correctly. I cannot see any reason why concert band could even be construed as "harder".
I am in marching band and concert band. Trust me when I say marching band is harder than concert band. In marching band you have to memorize music, where you march and you have to pass off or play your music from memory. In concert band you just have to sound good as a whole.
While there definitely is some effort in marching, the instruments are toned down to accommodate having to march. Not only this but a lot of marching is nothing but shouting with there rarely ever being an exception. Concert band requires a much huger sense of musicality. Concert band calls for more and is more demanding.