Cartographer and Forum are both doing similar things - sifting through your deck. In a very general sense, you could argue that Forum is better (not strictly better, just better), as it puts more cards from your deck into your hand, and allows discarding from both the new cards and old ones in your hand. However, I think it's worth considering:
- Cartographer has a reach of 2 more cards
- Cartographer interacts with your deck
- Cartographer does not require discarding
The first point doesn't matter too much, but the second is quite important. Cartographer can activate Mystics, Doctors, Vassals, Heralds, Wishing Wells... Now granted, a few of those could be argued as not mattering too much since Forum just draws the cards you would have been guessing with Mystic or Wishing Well, but being able to trash non-blindly with Doctor, and being able to set up Vassal and Herald chains, is quite nice. And if you don't have a source of +Buy, maybe you don't really want too much going into your hand, and would rather set up for next turn.
Anyway, I don't really know where I'm going with this, just something I was thinking of, particularly with a lot of talk during the last Qvist rankings about how Forum was a lot better than Cartographer, or that Forum even makes Cartographer obsolete. I think Forum is probably a better card in a broad sense, but if you do have the particular interactions described above, I think Cartographer does still have value.