ErictheRed wrote on 05/17/17 at 21:35:01:
One question to ask is whether you'd rather face the Catalan or Exchange Slav. You may think that 2...c6 is not so bad if it means not having to deal with the Catalan! That's one of the biggest questions to answer, in my mind.
Another is what to play against 1.d4 d5 2.c4 e6 3.Nf3 c6 4.Qc2, when if 4...Nf6 White can play 5.Bg5 or 5.g3. So the Triangle may or may not avoid the QGD and Catalan, depending on how you plan on handling those moves.
Dunno if you were responding to me exactly. But yes, whether to allow the Catalan or the Exchange is a very important question. Of course, objectively the Exchange Slav probably poses far less challenge than the Catalan.
But for me, I've played the White side of the Catalan for years so I tend to feel very comfortable as both colors in the position types that arise. I also value getting an interesting, dynamic position (after all, that's the whole reason to play the semi-slav rather than more solid QGD/Nimzo, etc.) which is substantially more difficult in the Exchange Slav.
As for the triangle with 3. Nf3 c6, it was always my impression that White had a variety of promising lines,
including the ones you mention.(Whether more promising than the pure Catalan after 3...Nf6 4. g3, I don't know). Hence 3...Nf6 is my preference.