It depends on what you are looking for. <br /><br />Google Sky is a 2D GUI essentially. You zoom in on an area of the sky and you see pictures that are enlarged and then replaced with pictures of a better resolution.<br /><br />Celestia is a 3D GUI. You can fly by objects, fly around them and observe them from any angle. The "naked" program lacks in true realism here and there but this can be easily remedied by using the add-ons that are abundantly available. You can find the links on the
celestia homepage. Here you can replace space object with the stuff users have prepared and some (not all) of it is actually quite stunning. Especially the 3D nebulae and high resolution surface/height maps add greatly to the realism. You actually feel like being at the helm of a spaceship that can travel at any speed to any place in the galaxy (and even beyond).<br /><br />My personal choice (3D maniac that I am) would be Celestia, but you must tweak the program a little with the add-ons, higher resolution planetary surface maps, etcetera.<br /><br />I don't know which one is more accurate. Celestia lets you enter coordinates of an object and fly there automatically or you can enter the name of an object. You can also fly to any place manually. During flight you can adjust flight speed from anything between a standstill and many light years per second. The interface lets you switch on/off several options, such as a graphical represention of trajectories / orbits, constellation names and shapes (interesting to see the latter change as you fly by), planet atmospheres. You can also add objects like the ISS, Hubble etcetera.<br /><br />Celestia is a free download as well. The open source character does appeal very much to me as it ensures a constant development. Warning: OpenGL compatible, fast video card is an absolute necessity.