If you look at resale values of a Canon and Leica of similar vintage and general price range - the M8 and the 1DMkIII - the M8 has held its own. Below are some numbers based on KEH offers to buy for a mint example of each (this provides a reasonable relative value, I believe):
1D MKIII - Orig $4000, KEH Offer (Mint) $1238, approx. months available 35
M8 - Orig $4800, KEH offer (Mint) $1530, approx months available 43
Depreciation:
Canon: 60% or 2%/month ($79)
Leica: 72% or 1.9%/month ($76)
So the difference in these two cameras is negligible. (But note that KEH is offering $300 more for a silver M8 vs black.) Of course if I had picked a different Canon, the results could be different. But in this comparison of the high end 1.3 crop offerings - the Leica can't be called a bad value.