The reason Apple is waiting till then for OLED is simply because of cost and ironing out any issues. OLED has always been the best screen technology and it's main claim to fame is it uses no backlight as all OLED pixels are self-illuminating. This means not more BLU (Backlighting Unit) issues and no more "spotlighting" problems. However, as with all technology there are also drawbacks, and for OLED these include:-
1. Lifespan - Degradation occurs because of the accumulation of nonradiative recombination centers and luminescence quenchers in the emissive zone. It is said that the chemical breakdown in the semiconductors occurs in four steps: 1) recombination of charge carriers through the absorption of UV light, 2) homolytic dissociation, 3) subsequent radical addition reactions that form π radicals, and 4) disproportionation between two radicals resulting in hydrogen-atom transfer reactions. However, some manufacturers' displays aim to increase the lifespan of OLED displays, pushing their expected life past that of LCD displays by improving light outcoupling, thus achieving the same brightness at a lower drive current.
2. Colour Balance - This has been the sticking point as most displays, including those use with Samsung seem to show oversaturated colours, and not not real-life accurate. It's akin to changing all your TV settings to max and it looks artificial / fake. Additionally, as the OLED material used to produce blue light degrades significantly more rapidly than the materials that produce other colors, blue light output will decrease relative to the other colors of light. This variation in the differential color output will change the color balance of the display and is much more noticeable than a decrease in overall luminance.
This can be avoided partially by adjusting color balance, but this may require advanced control circuits and interaction with the user, which is unacceptable for users. More commonly, though, manufacturers optimize the size of the R, G and B subpixels to reduce the current density through the subpixel in order to equalize lifetime at full luminance. For example, a blue subpixel may be 100% larger than the green subpixel. The red subpixel may be 10% smaller than the green.
Another important fact is some Samsung displays use a Pentile Matrix display system which is not as good as the RGB colour system. In effect the RGB pixels are shared amongst the rest which reduced image clarity and accuracy. Read more here:
http://www.oled-info.com/pentile
Excerpt "Samsung's Pentile matrix technology is a sub-pixel design architecture family. The basic PenTile structure is the RGBG matrix. In RGBG PenTile displays there are only two subpixels per pixel, with twice as many green pixels than red and blue ones. You can see a PenTile matrix vs a Real-Stripe one on the images below".
3. Power Consumption - the big caveat - While an OLED will consume around 40% of the power of an LCD displaying an image that is primarily black, for the majority of images it will consume 60–80% of the power of an LCD. However, an OLED can use more than three times as much power to display an image with a white background, such as a document or web site. This can lead to reduced battery life in mobile devices, when white backgrounds are used.
So all in all yes OLED is best however I do believe this over-saturation issue with colours which don't occur on LCD must be overcome and if a lot of your Apps / Games have white backgrounds, this will drain the battery quite rapidly.