hardwaremacbook properformancessd
I've tried finding the comparison reviews of the performance differences between 128GB and 256GB PCIe SSDs of mid-2013 MacBook Air 11" and 13" (which now have been carried over as the early-to-mid-2014 MacBook Air 11" and 13", too), but some comparisons don't even mention which SSD they've tested, and others only present a single size for the mid-2013 Air models.
How much faster is the 256GB SSD compared to the 128GB on mid-2013 MacBook Air?
The PCIe flash storage will be faster than the SSD, but how much you'll notice that in the real world is questionable.
The PCIe card used in the new iMacs is the same (custom) stick-like form factor Apple uses in their other products.
You should get the 1TB or 3TB Fusion drive configuration if you want more than 256GB of storage and don't want a larger SSD. You could of course get an external USB3 drive to provide large storage if you prefer, or even get a USB/Thunderbolt external SSD.
The $200 for the fusion drive is expensive, but not insanely so. It will give the most noticeable performance boost of any possible upgrade on the iMac, unless you prefer to spend the money doing an all-SSD setup, which will still be very, very fast.
You're not going to find a compatible PCIe SSD for any less than Apple's upgrade cost. For example, OWC sells compatible SSDs for the MacBook Airs (http://eshop.macsales.com/shop/SSD/OWC/Aura_Pro_Air_2012) that may work, but I don't believe anyone has specifically tested it.
I will also note that opening up the current iMacs is more difficult than in the past, due to the laminating process on the screen to reduce glare, parallax and weight. It would be a false economy to buy a $2000+ computer, only to risk damaging it by opening it up for the chance to save $50. If external storage is not desirable and you want more capacity, go Fusion.
It doesn't work
My Pentalobe and Torx bits arrived today and I tried swapping the drives. When I put the 256 GB drive in my Macbook Pro, it doesn't boot and displays this image
I tried holding down opt to boot to a usb drive and do a fresh install of Mavericks on the drive.
When I attempt to boot from the USB drive, the crossed out circle image is displayed again.
I've also tried holding down shift during boot to boot into safe mode. The neutral gray screen displays for a longer time, but eventually the crossed out circle appears again.
Best Answer
Sandisk/Marvell drives are about ~500MB/s write and Samsung drives are ~700MB/s write speed. Both have around 700MB/s read speeds.
It's important to note that while these are different, under regular use it's very unlikely that either will have any sort of performance implications. Benchmarks tell very little about day-to-day performance.