You are right. When you run out of RAM, your MacBook will swap some contents of RAM to disk. If the disk (backing storage) is an SSD, it is much less noticeable when this occurs than when using a HDD.I read somewhere that there isn't much of a difference between 8gb or 16 RAM for a MBP retina because it uses SSD.
RAM wise, they are exactly the same. That is how the software works with the hardware in Mavericks. From my tests, you will only need more than 8Gb of RAM if you are using a lot of VM's.Could it be because you are using an iMac and for MBP retina it might be not as efficient?
I'm not sure you can avoid swapping. Modern OS are designed to perma-fill up your RAM so everytime you open a file, an app, browse the web... etc something has to be removed to make room for the new data.The only difference is that on a notebook swapping also hurts battery life a little even if the performance isn't such a problem with an SSD. Which is why RAM compression is well worth it, which didn't exist before Mavericks. Usually in the use cases that you get to with serious swapping happening on a 8GB RAM notebook your battery life will be bad anyway because the CPU will be very busy. The bit of extra battery drain from the SSD wouldn't make a noticeable difference.
As has been stated above, your RAM needs will depend on your particular usage pattern. For the average OSX user 4GB is a little thin on the ground these days, 8GB is plenty, 16GB gives you comfortable headroom if you run a VM (like Windows) and is a good baseline if you're actually working in both OSX and your VM simultaneously, and you likely only need more than 16GB if you also need a Mac Pro.
I read somewhere that there isn't much of a difference between 8gb or 16 RAM for a MBP retina because it uses SSD.
Your not understanding the question (or the way RAM works). When RAM is exhausted (ie full), the OS writes some of the contents of RAM to backing storage. This process is called Swap (and also known as page-out). With PCIe based flash (used in all rMBP's), this process is around 10X quicker than a HDD and is much less noticeable than then when using a HDD. An SSD is hundreds of times slower than standard 1600Mhz RAM, but a hard drive is thousands of times slower than RAM. All this means that when RAM runs out (with RAM compression, it is around 15.5Gb where RAM is full on 8Gb of RAM), the affect Swap has on the user when using an SSD is much less noticeable than when paging-out with a HDD (which is crippling).Thats a lie since SSD speed and ram is 2 totally different things. If you are doing light editing, some gaming, web browsing and emailing then 8GB won't really feel different to 16GB. However if you have open loads of applications at the same time, play resource-craving games and do more heavy editing then with 16GB you will definitely feel difference.
And FYI, I have done heavy video editing (2hr long 1080p video, several layers of video, several layer of audio, lots of video stabilisation, several effects and transitions) with no problem at all. I have also had every app I have on my Mac open as well as iMovie doing some rendering, a PSD open, 50 tabs in Chrome, 50 tabs in Safari, a few word docs open, a few spreadsheets (large spreadsheets), a presentation open. When I done this, RAM compression allowed me to use 15.14Gb of RAM with my 8Gb of RAM. My RAM pressure was still great, and performance was still great. I have found with all of my tests that 8Gb of RAM can be pushed with around 15.5Gb of data before swap kicks in and RAM pressure goes amber. I think I could possible push around 16Gb before it gets too heavy though.Thats a lie since SSD speed and ram is 2 totally different things. If you are doing light editing, some gaming, web browsing and emailing then 8GB won't really feel different to 16GB. However if you have open loads of applications at the same time, play resource-craving games and do more heavy editing then with 16GB you will definitely feel difference.