getting more confused about SSD

Right now there are 3 common form factors for SSDs. 2.5", mSATA, and M.2.

2.5" is a drop-in replacement for 2.5" HDDs.

mSATA is a smaller size originally developed for ultrabooks created by adopting the mini-PCIe adapter used for notebook wifi cards. It is being phased out, but still pretty common. Especially as a secondary slot in addition to 2.5" - i.e. the notebook can take both a 2.5" drive and a mSATA drive at the same time.

M.2 is the official replacement for mSATA. It uses a new connector which can be used with a SATA interface or a multi-lane PCIe interface. So it can potentially be much faster than SATA, but the vast majority of M.2 SSDs out there still use the SATA interface. It also comes in a variety of sizes, with the 80mm size most common for full SSDs, and the 42mm size common for cache SSDs.

There are no 3.5" SSDs. The components are small enough they easily fit inside a 2.5" chassis. Any 3.5" SSD for sale is just a 2.5" SSD where they charge you an extra $20 for a $10 2.5-to-3.5 adapter tray.

I started out thinking I'd like a 3.5" SATA, but when I look at comparisons on this site and others, I don't see anybody comparing drives (or cards) based on a single form factor. I was more surprised to see that some sites have the top drives by MFRs that make memory chips, like Kingston or Sandisk or Samsung.

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SSDs use flash memory chips. So it's the memory manufacturers who make the vast majority of SSDs. Samsung, Micron (Crucial), and Toshiba make the lion's share, and supply flash chips to other manufacturers. Sandisk and Kingston make their own I believe.

The platter HDD companies (Seagate, WD, Toshiba, Hitachi (now split between WD and Toshiba)) had no technology which applies to SSDs, so they're behind. Toshiba was already in the flash memory business so they won't have any problems adapting. WD just bought an enterprise SSD manufacturer (a rather bad one, but hey any knowledge is better than none). I'm not sure what Seagate is up to.

The SSHDs do perform repeated read operations like booting almost as fast as a SSD. But anything involving writes or reading random files or reading lots of files goes just as slowly as a HDD. I do not recommend them. And in fact if you have a SSD + HDD and your motherboard supports Intel Smart Response Technology, you can allocate part of the SSD to cache, essentially turning the HDD into a SSHD.

After learning about the m.2, I see it goes into a custom slot on the mobo, and uses the PCI-E lines. I assume that means that even though I have PCI-E slots, I still couldn't put one on my board, right? (ASUS RAMPAGE III Extreme, no M.2 slot, unless I have it all wrong)

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Because disk benchmarks are measured in MB/s, it's tempting to think PCIe will give you a huge speed boost over SATA 3.

But your perception of speed is based on the inverse - wait times, or sec/MB. In terms of wait times, each doubling of MB/s only provides half the reduction in wait time. So the vast majority of the perceived speedup from switching to a HDD to SSD comes from SATA 1. A smaller portion comes from SATA 2. Moving up to SATA 3 gives you only half the speedup you got going from SATA 1 to SATA 2. And going to PCIe only gives you a quarter the speedup you got from going from SATA 1 to SATA 2.

So unless you regularly need to quickly read/write large files (e.g. real-time video editing), the difference between PCIe and SATA 2 is going to be almost imperceptible.

To sum up, I just wanted to get a fast and reliable SSD that I could put in a 3.5" slot in my PC case, preferably with SATA connection, either 256GB or 512GB, but not something that is pieced together in some kind of docking type structure.

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For a desktop, go with a 2.5" SSD. They're cheapest and you have the most options. 256-512 GB is the sweet spot right now, with the best performance. 128 GB SSDs are slower because they use fewer flash lanes, 1 TB is slightly slower because of longer memory lookup times. This will change with M.2 PCIe (512GB and 1TB will use more lanes become faster - the SATA 3 speed limit means there's no point adding more lanes right now), but those drives are still rare.

For a laptop, you're pretty much limited to what interfaces the laptop provides. 2.5" is still cheapest, followed by mSATA. M.2 is most expensive. So for that reason alone I'd try to stick wit 2.5" drives. Only go with mSATA or M.2 if it allows your laptop to run with a SSD + HDD simultaneously.