Did you browse the forum ?
There is plenty of thread here with feeback of both libs. Unfortunately, you may find difficult to make your own idea...
In both case, steinways are what these libraries has best to offer, and if you are only interrested in this piano, you really have to try them both, it's really a matter of taste.
- Ivory is less close miked, and has a natural flattering shining sound, great ppp and true una corda samples.
- Akoustik raw sound is less polished, extra close miked, and very very "in tune" (could be desirable or not, depend on your taste). Akoustik has the best "behavior" thank's to great harmonics emulation script.
German Steinway® D 9' Concert Grand, Bösendorfer® 290, Imperial Grand, Yamaha® C7 Grand
My personal favorite is the Bosendorfer, a very rich and warm sounding piano. Ivory is outstanding, very real and sensitive, though not always the best choice for the mix, but I give Ivory 2 thumbs way up. You have good control over the sonic shaping as well.