I almost hate to advise you, because opinions of libraries and their use as tools are so subjective. This is my opinion: the AO strings are not useful as primary strings. I would use GPO strings preferentially, faced with that choice. The GPO samples are better, and the Kontakt interface will give you a lot more expressive control with less work. This is one area about which I am, personally, convinced. I will leave it to others to comment on other areas of the orchestra.
But GPO will also give you a number of instruments not in AO, such as the piano, and the organ.
All I can say is, for 250 bucks, unless you are truly deciding between food and music, you cannot go wrong.
Frankly I can\'t see how you can go wrong with GPO, given it\'s low price. However it depends I suppose on how much you use it and how valuable your time is.
I think the primary benefits over what you have will be
1. legato control.. very useful for convincing lines and superior to the simpler keyswitching methods in your current libs
2. dynamic tone change with swells(as opposed to simply on velocity) In general this is much more controllable than prerecorded swells.
3. attack variation controls and bow alternation.
4. using Konkakt as a vst or dxi, rather than giga as a standalone. This was a huge improvement for my own workflow. Mixdowns are much simpler and faster.
5. If you purchase the full Kontakt, the tweeking of patches is easier than with Giga( in my opinion ) One can also save the instrument tweeks for a particular performance in a Sonar file.
6. One can generally load more instruments at once with a lower memory footprint and minimal dropout. The instruments are designed for maximum efficiency, with mono mode being used both for natural legato and note count reduction.
Also I think that just in raw sounds you will have a multitude of new sounds not present in your current libraries.