I've got both Vienna and EWSO Gold (and GPO).
GPO is easy to learn and easy to use and light on the system. The quality of sounds and articulations are on a much lower level, but you can get some surprisingly good sounding orchestrations out of it.
EWSO sounds excellent with some exceptions. Because Gold uses room mics it practically mixes itself. Panning is already taken care of and it sounds very realistic. And it already comes with a bunch of reverbs. There are a couple issues I don't like about EWSO. 1) the articulations are not consistent among a grouping of instruments. For example, you'd expect each string section to have the same articulations and they don't. What's more you're stuck with their programmed Xfades (which are also not consistent) and are forced to use more tracks to accomplish things which is annoying to me. 2) Play can be problematic (I've had my share of crashes). 3) Play is not the easiest to work with.
Vienna is superb although some of the EWSO samples are slightly better IMO. What I love about Vienna is the programming and ease of use. Each group of instruments have the same articulations. And the XFading is very flexible. It is super well thought out and easy to learn. The downside is that you will have to work hard to get a good sound with panning and reverb. It only comes with one reverb which isn't so good. It CAN sound great, but it doesn't sound great out of the box like EWSO.
I try to use Vienna as much as possible because of the ease of use and its rock solid performance. But I sometimes use EWSO for specific sounds that I like.