What is the difference between MathLink and WSTP? Which one should I use?

Question

What is the difference between MathLink and WSTP programs? Will the use of MathLink protocol be maintained or not?

Answer 1

There is no difference. "WSTP" is just a new name for MathLink.

As part of the renaming they also changed the function names from ML-prefixed ones to WS-prefixed ones. Now Mathematica comes with two developer kits, found in SystemFiles/Links/MathLink and SystemFiles/Links/WSTP. There is no functional difference between these two. The only difference is in the naming.

Note that at the same time when the renaming happened, a new version of MathLink/WSTP was introduced (interface version 4). Interface version 4 does introduce several new things, but it's important to understand that this is completely independent from the renaming. You'll still get the new things if you use the ML-prefixed devkit.

Personally I stick to the ML prefixed version so my code can more easily be ported to be compilable with the devkit that ships with v9.

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Update (2017 August, M11.1):

Which one should you choose for a new project, MathLink or WSTP? The documentation states that MathLink is deprecated in favour of WSTP (e.g. here).

Despite this, I recommend people choose MathLink, i.e. use the ML-prefixed functions. The reasons are the following:

• I am not aware of any contexts where using the WS-prefixed functions has a practical advantage (as of M11.1), but there are several cases of the reverse.
• LibraryLink still seems to use ML-prefixed variants, and won't work out-of-the-box with the WS-prefixed ones (at least not without any extra work—I never tried to get the WS-prefixed ones working)
• The CCompilerDriver packages always links to MathLink (not to WSTP). This package is useful both for compiling LibraryLink libraries and for compiling installable MathLink programs. It's especially useful for creating a cross-platform compilation setup.
• The Raspberry Pi version of Mathematica still includes only the ML-prefixed libraries, but not the WS-prefixed ones (verified in M11.0.1, the latest as of 2017 August). Your code won't compile on this platform unless you use MathLink instead of WSTP.
• Opening the Mathematica executables (either kernel or front end) with a hex editor reveals that internally they use ML-prefixed names (as of M11.1). This gives me some assurance that MathLink isn't likely to go away in the near future.

Needless to say, this is a personal opinion, and it is directly at odds with what Wolfram says. Follow it at your own risk. I am posting it here because it agrees with what Wolfram does.