GTX 960: 2GB or 4GB

It really depends on what games you going to play.. You will get a small performance increase in some cases
take a look at this video youtube

4GB on a GTX960 don't really justify the increase in price, since almost every time more than 2GB are in use, the card will be too slow to picture full details flawlessly. Also, for $250 there's the faster R9 290 available, so there's also that to consider.

I would recommend GTX 970 but if you decided on GTX 960, get 4GB one. Why? Many developers these days decide to implement 4k textures in their games and for lower settings they just lower the quality of some textures. They should implement 2k textures for lower settings but they don't want to. The point is, in near future you won't be able to set textures to high (not enough VRAM) and then on medium you'll see some bad quality textures.

On a personal note
Developers should change texture setting from ultra/high/medium/low/consoles (^^) to 4k/2k/1k, ultra = 4k, medium = 2k (everything would look good while nowadays medium = 4k/1k so some textures looks awful), low = 1k so everyone can at least play.

There are rumours about Nvidia reducing prices to better compete with new Radeons in power/price category.

2GB

Are you going to play on a 4K monitor > yes > then you need a better GPU anyway ...
but regardless the card GTX960 can't handle 4GB with that meager Memory Bus of 128Bit.

http://www.techpowerup.com/reviews/MSI/GTX_960_Gaming/29.html
^ check this out as well ... to get a glimpse of GPU's in the range of it

never be obsessed with memory as long it has low memory bus width like 128bit.
always better having card with bigger memory bus width (especially if you are into gaming) like 256bit and more!
this is really pretty bad card, maybe for some office work or something :)
If you looking for gaming cards move on and avoid 128bit cards!

If you have the money the 4GB, you wont see much of a performance increase, but it will help with games that have large textures or if you are using hi-res texture mods.

Depends on where you live (Israel?) and prices, market trending there, how often do you plan to upgrade... Sadly, some discounts never get outside of USA/EU and sometimes the market rules are totally different of what someone from the west could expect.

If you are planning to use the card for the next 5 years and can not afford to get a better one, then 4 GB of VRAM should be safer investment in a long run, as there were times when 1 GB was "too much for that card with that bandwidth" but then later all of those who got 512 MB had to upgrade while those with 1 GB had more time to decide on what to do even if playing in medium details, and the same happened when 1 GB was enough and nowadays it is not, even if medium-class cards from 2 years ago can play games like GTA V and Witcher 3 they would certainly be better off if they had at least 1 GB RAM more than they have.

I've had it with this uninformed silly meme "128bit isn't enough to use 4GB VRAM". I have a GTX 960 and it uses the whole 4 GB VRAM without any issues and slowdowns. Also the 128bit is only on paper, it's near 148-bit in real performance due to color compression technology. The GTX 960 runs every new game with 50-60 fps on ultra settings.

2Gb is the minimum you should get now, the sweet spot will be 3Gb, so I say 2Gb best option for a 960, if it was a 970 or 980 then 4Gb

Frankly… don't get a 960. Get a 970 somehow, if you must go for nVidia. The 960 is slow, the memory size of 2 GB will be a laughingstock in new games within a year. The 970 is probably their only competitive card on the market with a reasonable price for good performance.
Otherwise… 2GB. As someone already said, the memory bus on the 960 is just plain crap for its intended purpose.

Sorry for the off topic, but is the 960 4GB SSC any good? I got that as a prize from G2A but couldn't test it yet, need some parts before I can build a new pc

if you're buying a 960, then you're playing @ 1080p. You won't hit 2gb in that resolution.

Heaven's benchmark @ 1080p @ ultra preset and x8 AA will only use 1.2GB VRAM.
The highest I've seen is mordor @ ultra preset (except textures which is medium) and it used 1.7GB

check your motherboard and cpu compatibility with new card. maybe crucial thing for your gameplay experience.