If you do this, you'll probably later come to realize that picking such a large and powerful motorcycle as your first bike was a mistake. You'll have a harder time learning some of the finer points of control on a larger bike. The best riders usually started out on little dirt bikes, that's not a coincidence. I'm also speaking from experience. I learned to ride on a little bike, then very quickly jumped to a big one. I had fun, I loved it. It was still a mistake, because that big machine taught me some bad habits, and I didn't realize it until I went back to the small stuff.
Just something to think about.
To answer your question there are far too many technical differences between those two motorcycles to cover in a forum post. 2015 was a platform change year for the Multistrada, they share no major parts with the 2014 and earlier bikes, and among the changes was a variable valve timing engine (DVT). Very broadly speaking, the pre-DVT bikes have a more sporting character, while the DVT machines lean towards the touring end of things. That's a fine distinction, and largely opinion based, others might not agree. They are both very capable in either capacity, just a matter of focus. 2015+ has more advanced electronic rider aids, and an improved version of the semi-active suspension system both bikes have.
Personally, I would not buy a pre-2015 "S" bike, I would wait and fine a well sorted Pikes Peak. The early years of the skyhook suspension were problematic for quite a few owners.