Apple - 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/

Your issue is being caused by OS X's handling of a wi-fi parameter commonly referred to as roaming threshold. This parameter dictates the point at which an OS will switch to a stronger signal. Windows gives easy access to this parameter, example here, but no simple option exists in OS X.

This document from NYU suggests OS X has "aggressive" roaming characteristics and shows the command line to disable aggressive roaming. However, our experience on campus with OS X clients shows OS X to be anything but aggressive, insead preferring to "stick" to an AP it has associated with in spite of being in range of closer/stronger signals - like your situation.

So, there you go, there's not a lot you can do to alter OS X's roaming behaviour. What you can do though is alter your wireless environment in an attempt to force OS X to switch between APs. Options that you may have, depending on your wi-fi hardware, include setting your APs to strict 5GHz-only mode (if they are dual-band devices) - this will reduce the transmission range and therefore reduce overlap between APs. You may also have a setting on your APs for wireless transmission signal strength - this is another method for reducing overlap between APs, potentially forcing OS X to switch to a stronger signal sooner. Finally, you could try moving your APs to the extremities of your house, again to reduce their overlap.

fixed it by making the security identical