If you open 5/2 for witch/- there is actually a huge difference between drawing it t3 or t4. This difference is far greater then the difference between 5/2 and 4/3 openings.
If you draw it t3, you can buy something with your 6-card-hand, and then shuffle that and the witch back in immediately. You have a very good chance of playing witch for the second time on t5, and a small chance to play it again t4. Never worse then t6 though.
If you draw it t4, you're lucky if you play it again on t6, but t7 or t8 is also very realistic. t9 would be bad luck but possible.
And now I'm still ignoring the impact of curses, which only makes the difference bigger.
Suppose we both open 5/2, and we somehow magically are guaranteed to draw our Witch on t4. Then I would rather open Witch/Copper then Witch/-, to prevent the bad reshuffle from happening.
However, we don't get this magical guarantee at all. I could also draw the Witch on t3, and then I really really want that reshuffle. I'd estimate the upside of t3 witch without copper just about cancels the t4 witch with copper. However, the copper also doubles the chance that I draw the witch t5, which is pretty bad. And of course it adds a copper to my deck; in general a bad card and also I don't like early increases to my deck-cycle. I want to buy cards that are better then copper and I want to see them often.
Conclusion: opening Witch/- is much better then Witch/Copper, although sometimes on t4 I wish I could go back in time.
...with which witch I wish...