They convey the same information: Misa has long hair. But of course the delivery is slightly different. As such, emphasis in the first sentence is shifted to the hair. Maybe Misa used to have short hair, or maybe it is abnormally long.
Think about these two translations:
Misa has long hair
Misa’s hair is long
Same meaning, different emphasis. Your focus is shifted.
To add to what the others said, think of は as identifying what you are talking about (the topic). My high school Japanese class way back when taught to translate this as “as for”. However, I think in this case we can use “talking about” in a translation.
「みさの髪は長いですね。」
“Talking about Misa’s hair, it’s long, isn’t it?” (The conversation up to this point was already on Misa’s hair.)
「みさは髪が長いですね。」
“Talking about Misa, her hair is long, isn’t it?” (The conversation up to this point was on Misa, but not on her hair.)
You can also consider が as marking the word either performing an action (similar to a subject in English) or having a property. In this case, the second sentence marks 髪 has having the property of 長い.
Just something to think about as well: は is also used in situations where your are comparing things. So the first sentence could be talking about Misa’s hair in comparison to someone else hair. It could mean something like:
“Misa’s hair is long compared to other people’s hair”
Meanwhile, the second sentence is kind of just stating that Misa’s hair is long.
Thanks to everyone so far.
Seems it’s not as easy as I thought.
I will watch some more videos regarding this topic and maybe I’ll get the right feeling to use what in which situation.
I know it is hard to move on if you feel you haven’t fully grasped a grammar point, but maybe especially with は•が it is best to just let it be for now. You will encounter them so often that at some point you’ll get a feel for it.
Every once in a while look at a sentence in a book, or one you hear, and try to see if you can puzzle out why one was chosen over the other.
If one is particularly interesting/puzzling bring it up again and we’ll see if we can’t figure it out.