QGD vs Slav Defense

Which one is more solid and safer for securing a draw?

Do you mean mainline slav or semi-slav? also this question is too broad for a single thread, you can't generalize openings this way.  I can tell you the Slav is considered drawish, but you don't have to play it that way.  I have also heard that the Slav is only a potent weapon against an opponent that is trying to win.  I forget which great player said that.  If you want a great book on the QGD, you should check out Declining the Queen's Gambit, from Everyman Chess.  It shows some really great variations on formerly painful variations in the QGD such as the Exchange, that now are played more akin to the Berlin Defense, where black is doing well.

I meant the Slav Defense, not the Semi-Slav. Which opening is better if you're playing not to lose? Which one is the better drawing weapon?

In all the games I've played in my life, I've never lost with the semi-slav.

The Semi-Slav is sharper than the Queen's Gambit Declined and Slav Defense.

Id say the QGD is safer. 

Personally im not a massive fan of playing against the mainline dutch, which is a torture. Or playing against Geller's gambit, which is objectively 'equal' but very scary to play against, and practically I find better for white, since if black makes a mistake its GG.

Quick question: Have you ever played the semi-slav in your life?

Yes, I've done it eight times( 5 draws and 3 wins), both online and in tournaments.