Basically, when all said and done, the only differences you will notice between the two is weight and amount of dust left behind. Ive never used Birch for a sub box, but Ive worked with it at work before. I dont totally agree with the "it cuts easier, routes easier" stuff. But then again, I havent worked with MDF in over 3 or 4 years. I use Trupan. Here is some info I gathered up between Trupan and Birch awhile back (keep in mind Trupan weights approx 80% as a sheet of MDF.)
I gathered up some info on baltic birch 13 ply vs. trupan, and here is what I got.
Weight---
Trupan- 2.25 lbs. per square foot.
Birch- 2.16 lbs. per square foot
Price (my cost)---
Birch is roughly $.25 more per square foot.
The reason I compared square footage is because the Brich I have easy access to comes in 5x5 sheets (which sucks to handle) vs. 4x8 sheets.
In case your wondering, a 4x8 sheet of trupan weights in at 72 lbs and a 5x5 birch weighs 54 lbs.
Im gonna stick with using trupan. I dont feel the price is worth the very minimal weight difference you will get. However, if its wanted for a build, I would be more than happy to get it.