They are all great specialties that make lots of money, have great job opportunities, and a bright future. Also, are very competitive. If you are an IMG you can pretty much forget about getting a US residency in Derm or optho. You could probably get a spot in medicine, but I am not sure how hard it is to get the gastro fellowship. If you have specific questions about the residencies someone here can probably answer. And, you might not be an IMG, or might not be interested in US residency, I just noticed the Spain by your name.
Hope this is of some help.
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Rob
4th year med student at MCV/VCU in Virginia. Matched to Pathology at MCV.