I do not own SO, however, I am an owner of other products from EWQL, including Hollywood strings Gold and Hollywood brass Gold. I prefer VSL over EWQL for several reasons. For starters Vienna Instruments (VI) is superior to Play. It's intuitive, stable, resource friendly and flexible. Should you need even more flexibility and muscles, you can always upgrade to VI Pro, to benefit from humanization, auto voicing, time stretching, SSD-optimizations and more.
Regarding the sample content, VSL is recorded dry in contrast to EWQL. A matter of preference, of course, but I prefer my samples dry, and I'd imagine you won't find many forum members here with an opinion divergent from this. Furthermore, crossgrading from EWQL SO, you would benefit from true legato and portamento articulations.
What you would gain apart from this depends on which libraries from VSL you are considering.