In the Slav, after 1. d4 d5 2. c4 c6 3. Nf3 Nf6, what are all the differences between 4. Qc2 and 4. Qb3? In the Slav, after 1. d4 d5 2. c4 c6 3. Nf3 Nf6 :

What are all the differences between 4. Qc2 and 4. Qb3 ?

Which one of them is better to reach some Catalan type positions?

Which one of them tends to lead (on average) to the most solid, quiet, positional, slow, safe and strategic positions (i.e. the least sharp and tactical positions) ?

Is one of the two objectively stronger than the other?

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I have made some research and here is what I've found:

If Black plays 4...dxc4 then they transpose (except if White is crazy enough to try the rare gambit 4. Qc2 dxc4 5. e4 but I'm never going to play this anyway).

Against 4. Qc2 Black often plays 4...g6, while against 4. Qb3 the move 4...g6 seems to be less popular.

If Black plays 4...e6, maybe it makes a difference if the Queen is on c2 or on b3. Not sure.
After quickly looking at the differences, I think Qb3 is more suited if you want to fianchetto. 

The queen puts pressure on b7 and d5 and combines nicely with a fianchettoed king bishop.

If the queen is on c2 black often plays g6, planning to fianchetto and also play Bf5 with tempo. 

Both moves are equaly popular. The choice is yours!