what's the difference between "silicone sealant" and "mastic/MS-polymer sealant"
I found some information about it:

What is it exactly?

Look at this YT video (though missing in my opinion 2 great advantages: 1) much easier to work with than silicon (doesn't stick or dry too quickly), 2) huge lifespan once opened (it's hard to keep silicon for long time) ).


  MS Polymer™ is a water- and solvent-free, clear, odourless solution
  and liquid at room temperature. It has low volatility and a low Tg.
  When it comes into contact with a catalyst and water, a crosslinked
  matrix is formed, providing good heat and chemical resistance. 
  
  MS Polymer stands for “modified silicone”, although it is not entirely
  silicone-based. It has a polyether backbone with reactive silyl
  functionality (telechelic).There are 2 kinds: a dimethoxysilyl (DMS)
  with 2 reactive groups (each side) on it and a trimethoxysilyl (TMS)
  with 3 reactive groups on it, making it a faster curing system than
  the DMS type. The TMS polymer is a newer version which creates a more
  strongly crosslinked matrix during curing in order to make harder and
  stronger material.
  
  The biggest difference with products common in the PSA and tapes world
  is that crosslinking occurs through moisture-curing. So by addition of
  a catalyst and subsequent contact with water, MS Polymer™ will start
  crosslinking and form a matrix structure. Besides the functionality,
  which can be altered by using either DMS or more reactive TMS, the
  polyether backbone can also be tuned by varying molecular-weight and
  branching. The polymer is telechelic, having 2 ends, but also a
  single-end version can be used as a kind of plasticiser. The product
  range includes a high-strength grade, meaning that the functional
  groups are even denser, creating even harder or higher-strength
  material. A mixture of polymers can be used to make many different
  kinds of products.


What can you do with it?

This add another adventage to the list above:


  
  It's an adhesive.
  
  
  MS Polymer™ is mainly utilised in the adhesives and sealants world
  where it is a more common technology. (In contrast, silicone is not
  actually an adhesive, its adhesive qualities tend to break down much
  sooner than those of an actual adhesive *.) Its applications
  include sealants and adhesives for construction, automotive and DIY,
  such as waterproofing, glazing and insulating. 
  
  The polymer is also fairly to very compatible with many rubbers,
  acrylics, resins and additives. In other words, it is very compatible
  with many components currently used in PSAs to make a tape.


Some important questions remains:


is it as resistant than silicone ? (which can bear impressive amount of heat)
does it have the same lifespan?


source

If it wasn't more expensive than silicone, I would personaly use it instead since it's much better to use.