I like Marshalls better. TJ Maxx seems to have more of sophisticated/dressy line of clothing and Marshalls seems to have more of a variety of Juniors and Casual clothing. I like Marshalls prices and variety of other household accessories as well. The atmosphere is about the same in each store, just slightly different products. Although I have always felt Marshalls seems to be more brightly lit.
While Marshalls isn't usually as big as TJs, I always feel like Marshalls isn't as overwhelming and I always have amazing finds there. In my opinion they're both so similar it's hard to tell the difference sometimes.
I enjoy them both but the location really makes a huge difference. I went to a Marshall's in a town about 45 min away and it was AWFUL. However, the one near me is really nice & clean. Both are good, though!