Based on your IP address you will be directed to na.aiononline.com or eu.aiononline.com. But that will not determine on which server you will be playing Aion. If you purchased an EU key from the EU site yes you will be on EU servers. Your account will only work in the territory you purchased it in. If your account is not working perhaps you are trying to connect to the wrong territory (you choose it from the NCsoft launcher option where you can change the region between North America and Europe).
If you live in EU but want to buy NA version and can not because you are always directed to eu.ncsoft or amazon.co.uk you can:
- Ask your friend to buy you a NA key and you can apply it even at eu.ncsoft.com. It will recognize it as a NA key.
- Or you can use a third party company to buy a NA box
There is no difference between the NA/EU client. Clients are identical minus region specific language packs.