I don't think you can say any particular local Orthodox church is particularly welcoming/unwelcoming to converts from outside their ethnic group - you just have to go and see what they're like. In my own experience, I've found that the Greeks are a little too Greek - they welcome you but they try to Hellenize you also - others here haven't had the same experience at all, though. Personally, I've found the Romanians, Russians and Antiochian parishes to be very welcoming but, again, others may have a different view.
I would like to point out one thing, though. If you're seriously thinking of converting, you really need to figure out whether you agree with Eastern Orthodox or Oriental Orthodox Christology first. It just seems a really odd thing to me to that you appear to be considering the Greek (EO) and Coptic (OO) churches as though they have the same beliefs - but then I'm Romanian Orthodox, so fall firmly in the EO camp.