basser, I don't think it matters whether you buy MAE.to or MNG, if you are purchasing in US dollars. In fact you would probably be ahead purchasing MNG, due to losses via the exchange mechanisms.
These two liquid issues should always nearly reflect an approximate equivalency, via the current exchange rate. If the US dollar continues to fall, and MNG/MAE.to continue to rise, MNG should rise more, in % terms.
Illustration of Jan 2002/Present Prices and Profit Ratios:
MAE.to: 3.34/.78 = 4.28
MNG: 2.54/.49 = 5.18
The difference between 5.18 and 4.28 is due to the falling dollar, relative to the Canadien dollar. A purchase of MNG in US dollars takes advantage of that quite nicely.
I'm a large holder of MNG by the way.