For PCIe SSD, they are very good for dealing with very large files. e.g. copying videos, unzipping huge files, loading a VM, etc.
For office, gaming, web browsing. There is not much difference between PCIe SSD and SATA SSD. 100% sure the difference is nowhere near the improvement from HDD to SSD.
I will do some photo / video editing as well. And this is my SSD usage history.
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Despite the SATA II speed limit is around 250MB/s, and my SSD can run at 250MB/s (confirmed by benchmark). My real world usage rarely hit 50% of the bandwidth over the last 7 days. That's including some photoshop, and simple video editing. So, in my own workflow, even SATA II is not a limiting factor for me. Moving to PCIe SSD is more for fun if I do that.
Reality is, PCIe SSD only able to achieve that crazy speed in sequential read / write. So, if you are not dealing with huge files regularly, you can hardly benefit from a PCIe SSD (AHCI).
For small files read write (e.g. 4k random read), there is only minor performance difference between a SM951 and a 850 Pro. Both are doing <50MB/s (for your info, HDD is about 1MB/s, that's why you feel the significant improvement from HDD to SSD. It's nothing about the sequential speed, but random read speed. Because OS are formed by thousands of small files, but not a single huge file)
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In any case, PCIe SSD is faster, but most likely not that much as you are expecting (depends on your workflow). If your SSD history shows that you regularly hit the 250MB/s limit, then a SATA III card or PCIe SSD will help.
Since the PCIe SSD adaptor is nothing more than a pure adaptor (no controller on it), that means any adaptor should able to do the job, just different in price, looking, quality, etc. For SSD, SM951 (AHCI) is the best you can get. However, installing Windows on it is tricky. Only few members here report success, quite a few goes into different kind of trouble. Since there is no significant benefit by "booting" from PCIe SSD, or even from a SATA III card, so, I personally won't recommend anyone to do it. Booting from a SSD that connected to the native SATA II port is the most trouble free solution, boot time is more or less the same, apps loading time may be just half second more. So, store the OS and apps on the SATA SSD, and install another PCIe SSD for very heavy duty make more sense to me. The space on PCIe SSD is valuable, storing OS files / apps is not the best way to utilise it.
e.g. For VM, I will choose to install Parallel on the SATA SSD (same SSD as the OS), but the entire VM file will be on the PCIe SSD. Or FCPX is on the SATA SSD, but put the 100GB 4K video file on the PCIe SSD for editing.
As a general rule, PCIe SSD is good for everything, but since the price is high, and the size is relatively small. So, better to use it for very large files. If money is not an issue, go for it, it's no doubt the best hard drive.
SATA SSD is good for most stuff, not as fast as PCIe SSD, but small files read write are good enough for OS, apps, photos. By considering the price is much lower than PCIe SSD and the size is larger. It should be the optimum solution for most normal computer user.
HDD is good for providing storage space. It's cheap. In fact, it's quite good for medium size files (e.g. 100MB). By considering it's sequential speed is usually >100MB/s. Loading medium size files still just a matter of second. However, dealing with small files is horrible (because of the latency from the mechanical movement), we should let SSD to take over this job. And deal with huge files also very painful (e.g. it can take >20min to just copy a 150GB video file, which a PCIe SSD only need <2min)..
