I created a wallet @ myetherwallet. I received some ethers here and it worked fine. But I didn´t understand where the ethers were at first, so I openend another wallet locally on my mac: ethereum wallet. It shows 0 ethers when I import the same keystore file I use with myetherwallet online. I bought some new ethers now, in which wallet will they show up as they are being sent to the keystore address that is used by both wallets?
A precise way to answer this would be: The balances will be in control of anyone who can identify himself to the network as the account holder, through a key file, for example. So you can control all your funds of your accounts wherever you add your key.