Both are definitely playable. Berlin is a library which is more feature rich, has more articulations and so on, and Afflatus is configurable in terms of number of players, as descried in another post.
If you want to create mockups not just as a part of your compositional process, but in order to use for published work, Berlin will take you longer, and has portamento, different vibratos which can be switched between (bot not crossfaded between on the fly out of the box; your vibrato change will be valid from your next note), attack control, fast runs (which I think I read somewhere that Afflatus may get later?) and so on. But if you want to use one of these libraries mainly as a way to compose and get some inspirational support from what you hear, Afflatus could be a better choice, at least for some of us - but read on: Berlin also has some inspirational presets.
They both come with a lot of presets actually, so you'd have to look up each of the libraries and check what I said about number of articulations. Berlin comes with many short note choices, a number of long note choices and also many dynamic longs, or 'arcs' - and is a more 'logical' library while Afflatus may be at least as inspiring, especially the Lush and Scene d'Amour presets.
Berlin may be a better library, but if I could take only one of them with me to some remote place to compose, I'd probably choose Afflatus, unless someone wanted me to write something with lots of fast runs and also hear a mockup with all kinds of shorts/longs/arcs.
Berlin Strings comes in a 8-6-5-5-4 configuration, while Afflatus can be layered with itself to get everything between 2 and 15 players (I checked this with the violas, maybe the V1s have even more options?).
"Afflatus is particular with the legato"... true, but all longs in Berlin can be made legato (although not full adaptive legato as their main legato banks are - check YouTube for Berlin Strings legato), and Afflatus also have some very alive sounding shorts.
With A you may get extra stuff you may or may not need, and this extra stuff may also have increased the price of the product. This is true for B as well, but Berlin comes with longs/arcs/shorts that you may not need now, but maybe will need later. And, if you'll go for the Berlin expansions A and B, you'll also get access to their Sul Tasto Sustains (soft/immediate) presets which may be the closest you'll get to Afflatus' Lush/Scene d'Amour presets. The Berlin cellos and viola soft sul tastos are brilliant (but then again, I generally think violas and cellos are brilliant!).
I have found myself sitting down playing and getting ideas more with after my first meeting with Afflatus than I did with Berlin, at least before I discovered Berlin's 'Whole Ensemble Sustains Soft' in the main collection. The individual Berlin Espressivo and and Soft Sustain presets in BS are also very good presets - which 'breathe'.
With Berlin you may get a more mature library - with more options, especially if you go for the expansions as well, and good 'adaptive legato' presets which gives you a distinct attack if you play a note with high velocity, and a soft attack if you play soft. Afflatus may need more switching between presets (disclaimer: I've had Berlin for years and Afflatus for a couple pf days).
Since I don't know if you want to mainly compose or create as good mockups as possible - or know what kind of music you want to write, I can't give any proper advice, of course. They are both very good libraries, but in different ways. If you want to control section sizes, have polyphonic legato and divisi, maybe Afflatus is the best choice for you, and if you want to control details and a more 'classical' product with portamento and more vibrato/attack control, Berlin may be your thing.