Manuals:

Choose which type of connection is more suitable for you, whether L2TP or SSTP and choose the instructions on how to set up your device accordingly.

Main differences:

1)Ease of setup

L2TP connection is natively supported by all Windows, Apple-iOS, Apple-MacOS, Android devices. So that is easy so set up.
SSTP connection requires a slightly more complicated procedure of installation into the device, in most systems you have to download the application as an intermediary, but the use is still reliable and completely stable.
Once you have set up a VPN, there is no big difference in usage.

2)Versatility of use

L2TP connections can be actively blocked by your ISP or authorities.
SSTP connections are very difficult to block, it is practically impossible (exceptions are blocking at the level of target IPs).

Examples of use and decision making:

1) Connecting to a VPN due to audio / video communication with foreign countries in countries where this communication is blocked (UAE, etc.) - L2TP connection may not work everywhere, typically the Etisalat operator restricts this connection, select SSTP, that's for sure.
2) 2) Connecting to a VPN due to broken e-banking or some sites where you are registered and which do not work without a VPN at all or badly - L2TP connection almost always works fine, exceptions exist again, select L2TP and only if it does not work, remove the L2TP VPN from the device and try SSTP.