Heavyocity Gravity or 8dio Agitato bundle.

They're both about $280.  I write all styles of music and the agitato (I think) would really liven up my strings pieces.  But Gravity is the grandaddy of versatility.  Which would you buy if you can only buy one?

Now that the dust has settled, I've not heard one person who was truly happy with what they spent on Gravity or how Heavyocity has handled the plugin and its expansions.

Just stupidly deleted a half hour response!  OY! My dog Volts  been in the pathfinder waiting.... heading to courdelane...don't have time to do it all again... but...to summarize:
 
Gravity... NO Get expansions when they are half price.  whoosh/bangs ya want???  Daniel James products bundle:  Alpha/Bravo
8dio... emotional... thats what I love; got lots... and yea, good deal but.... but....  but for orchestral, choirs, cinematic drums, drums and guitar slams.....
 
The best BF deal this year:  100Gigs from one of the best developer out there, Orchestral Tools: Metropolis Ark 1.... blows everything else away for what chu get for your dollar... and I've spent a zillion!!!
 
http://vi-control.net/community/threads/black-friday-2016-metropolis-ark-i-the-monumental-deal.57366/page-2#post-4017955
 
Original 24 page thread by lots of VERY amazing composers orgasming for REAL over at VI Control: http://vi-control.net/community/threads/orchestral-tools-metropolis-ark-i-released-new-intro-special.50203/
 
Get that... if you don't have it...  compared to ARK, both your other options are suboptimal....
 
I did a way better job on my first version.... Good by all, 
 
Enjoy ur holidays!  
 

Get Vocalise at AudioDeluxe for $41.
Edith: or Gravity for $190.

I don't have Gravity, so I can't comment on it. But I do have Vocalize, and I like it a lot, and it sees a lot of use. I do cinematic/orchestral stuff pretty much exclusively, and Vocalize fits the role perfectly. If that is the sort of music you want to make with it, I think its a no-brainer for 40 bucks.
 
I just bought Agitato (not the whole bundle, just violins, violas and cellos grand ensembles) and Adagietto, and I REALLY like them. Gorgeously emotional, and not subtle in the slightest. However, all the usual complaints about 8Dio's stuff should be kept in mind: there is some panning oddness between different samples; not all the samples are clean or artifact free; and the legato slurring takes some practice to play, a lot of post-recording tweaking to really shine, and it is occasionally glitchy.
 
But when they work, they really damn-well work, and I've been thrilled with the results I've been getting with them. I'm not using the 8Dio libraries as primary string libraries -- I don't think that's their real purpose -- but instead as killer accessory items to my other full libraries. So far, I've layered them successfully with Albion, Da Capo and Hollywood Strings. They are not for everyone -- hence the rather polarized reputation they have -- but if you take the time to learn how they work and what they do, I think they're an essential addition to a classical/cinematic library setup. And you can't beat the current price.
 
Just my two cents.