Setting a Preferred Access Point

I've got an Airport Express that is wirelessly extending my AT&T U-verse WiFi network, and providing a network interface for my ethernet (but not wifi) enabled printer.

Prior to the update with 10.10.4 beta yesterday (that promoted mDNSResponder in place of discoveryd service) I was also using it to connect my iMac via ethernet over the wireless bridge (the iMac was dropping it's WiFi repeatedly due to discoveryd flakiness).

All the networking works fine, though the bridged connection is slower than the direct WiFi connection from the iMac to the U-Verse gateway. Since the WiFi connection on the iMac is now rock solid I want to force it to connect to the U-verse gateway rather than the Airport Express that is physically closer (and as a result has the stronger WiFi signal).

I don't really need the WiFi to be extended as the signal is strong enough throughout the house for everything, so I'm wondering if I can either

A) Simply make the AE a wireless client of the U-Verse network and stop it from extending the Wireless network

or

B) force the iMac to connect to the Uverse gateway as a higher priority Access Point

I DO NOT want to put the AE and Uverse gateway as different wireless networks (and I don't think I can since it's connecting over WiFi anyway...)

I have explored the "joinMode" setting of the airport command line utility, but it also seems to be SSID preference rather than AP my MAC address for example.