Just because SD is a solid state technology does not make it anything like an SSD. To compare the two is to compare floppy disks to hard drives. The controller is the primary reason for the performance difference, but it goes further than that. SSD's have extra capacity and functions in the controller to account for memory that fails, SD controllers don't because SD is not made to have dozens or hundreds of writes per day, it is made to store a file and that is about it. This is one reason windows is so bad on a slow storage device - there are dozens of reads and writes every few seconds that have to be done to make Windows run. If those are slow, the OS will be slow.
It was mentioned already and I would second it - SSD's have come WAY down in price in the last year, and the smaller ones are super cheap and literally 20 times faster (which will translate to an extreme performance gain, especially in any windows based OS). Unless you were simply given the adapter and a high capacity SD card for free, there would be no reason to choose an SD over an SSD.
Just because the adapter is using the SATA bus doesn't mean it will run at SATA speeds. Just like your CD Rom drive won't run faster just because it is on a SATA bus. Expect faster performance from the SD adapter than you would get with a CD Rom, but slower than a regular mechanical HDD.
As for size, 3.5" disks were the standard for a long time for desktops and servers, but in the past couple years laptop production has surpassed desktop production and is now substantially ahead, which means that any technology specific to a desktop is starting to climb in price, where any technology that can be used in a laptop has been going down in price. Hence, the 2.5" hard drive format is more prevalent today than the "large format" drive. Keep in mind though that 3.5" is still a better way to create large capacity drives, so if you want a cheap 3 or 4 tb drive, you go 3.5". All ATX cases made today have at least one place to mount a 2.5" drive, and just about every 2.5" "retail" drive comes with adapter brackets if you want to mount in an ATX case. Bottom line is that today, 2.5" would be "standard".