I would imagine GPO, because GPO is a complete library. The Giga VSL is ment to tease you and make you want to buy the whole library. I would recoment mixing the two. GPO doesn't have the best trumpets, so try the VSL trumpets and see how they compare.
It's all one big experiment.
Seems a stupid answer but why don't you install it to find out!
Here's why -
I'm not a composer. I don't do soundtracks. I don't work with these libraries on a daily basis. But I have a keen interest in these things, hence me being a daily reader of this forum.
I play guitar in a heavy metal band. We're signed to one of the biggest indies in the genre and we're working on our 8th album.
I track in Colorado but we mix in Florida. On several occasions I've spent lots of time working out sample parts here in Colorado only to have them rejected by our producer in Florida. And he was right, for example the orchestral instruments on a 8 meg Proteus sound nothing like the real thing. Today that's a no-brainer, but back then it was easy to be seduced by the Proteus.
I just don't work enough with strings to have the ear for it. I'm trying and I'm getting better. But there are people here that work with these libraries on major motion pictures. They work with these samples daily.
That's why I ask the question. I want the opinions of those who are much more qualified to answer the question.