What's best for you depends on what you want to use the library for.
I've got GPO4 and Miroslav. They're in the "affordable" range - certainly a large step up from GM and free SoundFonts.
GPO gives you a nice collection of instruments, and they fit together well. They are recorded dry, but come with reverb, which really helps create a more "orchestral" sound. There are some nice woodwinds, but the strings are only OK. The solo strings and brass are a bit of a disappointment. (Good reverb, which GPO has, helps a lot!)
Like any other library, if you work with it you can get
very good results. If you've got any doubt about that, have a
listen to some of the Christmas songs. Although I was pretty unhappy with the solo strings, but after digging into the MIDI controllers, I was surprised that I was able to get much more playable instruments than I had expected (these are all experiments with articulations, so ignore the cheezy muzak. The melodies were auto-generated by Band in a Box, and I edited the MIDI tracks in Reaper):
GPO Solo Violin Test
GPO Solo Cello and Violin Test
Not
great, but certainly passable in context. Of course, a nicer library is going to give you that without making you work so hard at it!
GPO is also pretty lightweight in terms of CPU, which is nice.
Miroslav is an older library, but it's got some very nice sounds. In comparison to GPO, I find the instruments to be more 'detailed', and there are a lot of different articulations available. In general, I think that the sounds are better than GPO, but it's also a bit heavier, and takes longer to load. (I suspect the age of my machine is a factor).
Here's a test I did with the Miroslav violin:
Miroslav Violin Test
I'm using about 6 different violin articulations there, each on a different MIDI track. It took a while to put that together, but there was a lot of learning involved!
So again, if you take the time you can create a compelling performance - it just requires a bit more work. As with any library, you'll want to write to the strengths of the instruments and downplay their weaknesses.
Is there anything in particular you were wondering about either of these libraries?