Need help to choose between M.2 or SATA 3 SSD

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@SyedAdeel wrote:

I have a dell inspiron 5567 with 1TB mechanical hdd. I am planning to install an ssd of 128GB and set it as boot drive. Now as this laptop do not have M.2 slot, i have an option of using a M.2 caddy to install M.2 SSD in either optical drive slot or internal hdd area. I need suggestions to choose between M.2 SSD ( like Samsung EVO 960/970) or a 2.5'' SSD ( like Samsung EVO 860). In either case, my selected drive will be used just for booting. Keep in mind, I dont have a M.2 slot, i will use a M.2 caddy. Thank you in advance :)

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Your plans have an even bigger problem than @ejn63 mentioned above.  The Samsung 960/970 are M.2 NVMe SSDs.  You cannot use those through an M.2 to SATA adapter.  You could use an M.2 to SATA adapter to run an M.2 SATA SSD, like the Samsung 860 Evo in M.2 form factor, but it will perform identically to using the same SSD in 2.5" form factor.  M.2 is a slot type, which can support the older SATA and/or the much newer NVMe data interface -- which is why there are both M.2 SATA and M.2 NVMe SSDs.  To my knowledge, all M.2 motherboard slots that accept NVMe SSDs also accept SATA SSDs (for now, anyway), but an M.2 slot that only accepts SATA SSDs will NOT work with NVMe SSDs, and if you're connecting an M.2 SSD to a SATA connector, you definitely can't use NVMe -- and even if you could, it would be severely bottlenecked by the SATA interface anyway.

I can't independently confirm or refute whether a hard drive in an optical bay will be bootable -- my guess would have been yes since after all it's possible to boot from an optical drive installed in that bay, but ejn63 may be speaking from personal experience there.