Mac mini 2012 (2.3 quad core) Vs late 2014 (2.8 dual core)
Go with the older model.

The quad core i7 can run circles around the dual core i5 in multithreaded applications, which Xcode is (especially for code compilation tasks). You've seen that on the Geekbench scores.

The hard drive's speed is not really going to make much of an impact on your intended use.

The graphics chip is completely irrelevant.

The additional RAM will be a big boost especially running Yosemite.