Why is it easier to measure overall structure of Andromeda Galaxy than to measure Milky Way Galaxy?
Because we can see the whole thing in visable light, and take neat photos, while it takes Radio and IR imaging to see through the dust and gasses blocking our view of most of our own Milky Way. Because we have to "see" our own galaxy edge-on, no neat spirel structure; we are in it.
It's like holding a book right up to your face edge on, you can certain aspects of it's structure quite cleraly but you can't really see the bigger picture, add to this all the interstellar gas around the galactic nuclei which makes that region very difficult to observe.
Welcome to Physics Forums, BFern!
Note that it's easier to see Andromeda's overall structure, but harder to see the internal details (because it's so far away).
To add, and just for someone to look up if interested, somewhere on the web you can find an unbelievable photo of the giant elliptical galaxy M87. They say our Milky Way has about 150-200 Globular Clusters (I think more), but the photo of M87 shows good detail in the galaxy and ~10,000 (yes, ten thousand) globular clusters in the halo.
EDIT: Here it is: (larger photo would be nice)
http://www.seds.org/messier/Jpg/m87.jpg [Broken] From:
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