How to get OSX to pick the strongest wifi signal with identical SSIDs?
I know this thread is old, but I'd like to leave here what solved the problem for me. You need to check if you have the same security in both access points. For example, I had one access point secured with WPA/WPA2 and another with WPA2 only. After changing both to the same, everything was fine.

To check your access points' security, you can do this:


While holding the option (alt) key, click the WiFi icon in the menu
bar
Click "Open Wireless Diagnostics…"
Go to "Window" > "Scan" (or press CMD-4)
Check if the security is the same for both your access points


Solution source: http://forums.macrumors.com/threads/wifi-roaming-issues-two-aps-with-same-ssid.1208584/