I've been to the Desert Inn several times in the past few years, I usually get the Parlor Suite with spa. It's on the 5th floor.
I think, but am not sure, that the parlor suites are on the 4th floor.
There are two elevators; the main one, which is located at about the center of the building, and another one at the west end, kinda by the lobby. The west one only goes to the 3rd floor, and not to the parking garage. There are stairs nearby that go to the 4th and 5th floors.
It's not a very long walk from any room to the main elevator, and it goes from the parking garage all the way up to the 5th floor. There are stairs that go up to the observation deck, where you can watch the fireworks.
I don't think there's much difference between any of the rooms, the ones I've stayed in have been pretty quiet, even if the hotel is full.
I've sated there a couple times, different rooms each visit, and I have nothing but great things to say about it! I don't really think it matters what room you end up w, they all seem really well kept and nice! I agree with the noise factor too, there was a wedding reception the first night we were there last time...couldn't hear a sound
We stayed at the Desert Inn and Suites last July and everything was fine. It's not the greatest hotel in the world but it's clean, easy walk to the parks and cheap. We had the parlor suite and might try the C1 suite (for our next visit) which has two rooms connected.