Thanks everybody for your answers! I have been reviewing my results with the Classical Slav and overall I am very happy with it. I even have a good score against the Exchange! The only problem is when white is dead set on a draw, but I think that is the case with most black defences. And otherwise I can occasionally play the Leningrad or the Benko again!

As usual, I will say that it depends. What type of player are you?

Semi-Slav (or even more extreme - triangle slav!) is extremeley sharp, which means that if you know your stuff and White doesn't the chance is high that you emerge with a winning or very good position. Seen that a lot of times, people go down the main line without knowing anything. 
However, this means that you will need to work off the board to be ready for everything. Work and reward are high!

The normal slav is another beast. It obviously leads to calmer play, without being dull (maybe a bit) I would say that chances of losing are lower than in a semi-slav that is not properly learned. You need to work at the board, which means outplaying your weaker opponents in Endgames and so on. 

So, figure out wether sharp, dynamic play is yours or if you prefer having a solid position and playing from there. If you tend towards the former and have the time, absolutely go for semi-slav or triangle! The reward is just so high! Don't be lazy if you do not have to

Hope this helps!

It probably is, but it's also a better way to lose.  It's more sharp in general, and many lines have Black surrendering the center early on to cling to an extra pawn on the queenside, which means that in practice White's play can be easier than Black's.  Think about the Anti-Moscow gambit.  Even if White isn't well prepared, he has a pawn center and his king is castled, while it isn't clear where Black's king belongs and he's advanced pawns on both flanks.  Personally, I wouldn't play that way against a lower-rated player.

It really depends on your own style, to beat weaker players you generally need to play into your own strengths while giving them plenty of room to make a mistake.  You don't want them playing simple positions with a clear plan against you, and you don't want them rattling off many moves of theory against you (unless it lands them in a position that suits you).  You want them making decisions on their own.