Both of these have been thoroughly tested and are in active use in various labs. They use the same underlying API to communicate with the probes, so their functionality as far as acquiring data should be identical, though they differ in their online graphical display, interface for modifying probe settings, options for online data processing, and file formats for saving data.
SpikeGLX was developed by Bill Karsh at Janelia Research Campus. Use the "SpikeGLX_NISIM.exe" version if you do not also have a national instruments acquisition board attached to the Neuropixels acquisition computer.
The Open Ephys GUI is an open-source, plugin-based application for extracellular ephys data acquisition. It was developed at MIT by Josh Siegle and Jakob Voigts, and is currently in use in over 100 labs around the world.
A plugin for communicating with the Neuropixels basestation is available in the “neuropix” branch of the GUI. Although the GUI is cross-platform, the Neuropixels module only works on Windows. It's only been tested on Windows 7 so far.
If you know C++ (or are willing to learn), there are many opportunities to contribute to the GUI source code. For more info, please contact joshs (at) alleninstitute (dot) org.
For instructions on how to configure Open Ephys for use with the Neuropixels probes, check out this page on the Open Ephys wiki.