Buying Hot
Where you live. But don’t be surprised if your game gets banned. G2A is famous for selling games bought with stolen credit cards. Don’t be cheap and buy it from ArenaNet.
Please, don’t buy from a shady place. That is asking for trouble. Also, there is no difference in the HoT key depending on either where you live or what server you play.
Buy only through Arenanet. Any other path may get you banned.
The only difference in versions between EU and NA is the default currency shown when purchasing items/Gold with real cash. NA defaults to US Dollars, EU to Euros/Pounds/Etc.
Good luck.