Personally I think having taken the USMLE was very helpful during my application process. It's not so much that they discriminate against DOs but the programs will be able to compare apple against apple if you have the USMLE scores. Some programs will use scores as the initial screening tool, some directors or program assistants may not spend the time to learn the scale on the COMLEX. Thus, you might not get interview invites simply because they are not familiar with the COMLEX. Having said that one of my classmates only took COMLEX and she interviewed at places like Jefferson, U of CO, USF, and quiet a few other places. She ended up matching at USF. So it is possible to match only with COMLEX.
If you are shooting for Ivy League places, USMLE is highly recommended.
As far as 4th year externship, I did my neurology rotations at Barrow Neurological Institute and Penn State. BNI is an incredible place. They do a lot of cutting edge stuff, have awesome neurorad and neuro surg program. The stroke program is very strong. Dr. Frey is a phenomenal neurologist and down to earth guy. You will see a lot of zebras and yet still have good number of bread & butter cases. You will learn a lot there. PSU is a good place also. You will mostly see the bread & butter cases. My advice is to do a rotation that get you a good foundation before going to places that have lots of zebras. Reason being that at high power places like BNI they will expect you to have the basics down. They generally have higher expectations.
Hope this helps.
