As far as stock string sounds/patches, I'm fairly impressed with the older Yamaha Motif 8 (classic) I just purchased. I know newer Motifs have bigger ROM sizes for waveforms, but I don't know how strings sound (at least first hand) on them.
I use Roland stuff too (XV-3080), and have always liked their Orchestra SR-JV80-02 expansion board string selections. This of course is a separate purchase for the expansion, but worth the $$$. These could be found on eBay these days. They do have the newer SRX-06 expansion which includes all the waveforms from both of their earlier Orchestral expansions. I had an Roland XP-80 in the late 90's with this expansion, and later purchased the Orchestral II expansion, both served me well had a very nice variety of quality samples. For newer Roland stuff, maybe someone has heard the Roland Fantom strings selections? I haven't yet.
The Kurzweil Orchestra ROM gets favorable opinions as well. This is included with at least one of their keyboards (if not more), otherwise it's an optional expansion. I know there are Kurzweil users here, so I'll let them chime in on the quality and better details. I've heard MP3 demos, and like what I heard, but I like to hear stuff first hand and play it for a true idea of quality. By playing the sounds/patches, you get a true idea of their expressive or non-expressive nature.
Well, there are options for you from three different manufacturers already. In conclusion, I love the Roland expansions, and have a new found fondness for at least a handul of stuff I've heard on the older Yamaha Motif.
Good luck. I'm sure many others will chime in, and you'll be more confused about which one to get as ever...
Just kidding!