Get an Alfine while you can, who knows when there'll be more IM9s here in the US. I made a spreadsheet with gear inch charts for the Alfine vs IM9. The only real difference is that the IM9 has an extra high gear. The IM9 is constrained to a primary gear between 1.7 and 1.9, this sets the range.
IM9 with 1.7 primary gear (34/20) gives a range from 27 inches to 90 inches.
Alfine with 1.7 primary gear (34/20) gives a range from 26 gear inches to 80 gear inches
IM9 with 1.9 primary gear (34/20) gives a range from 30 inches to 102 inches.
Alfine with 1.9 primary gear (38/20) gives a range from 29 gear inches to 89 gear inches
These are the limits set by the SRAM with 29er wheels. I don't know what Shimano's limits are.
Alfine has thumb shifter, IM9 only has grip shift. Alfine w/shifter cost $250, IM9 w/shifter cost $450. That's a big difference in price for an extra high gear, esp since a MTB doesn't have much use for the extra high gear. I've purchased both the IM9 and the Alfine, I'm going to give both a try and decide which one shifts better. If the Alfine shifts better too...I might have an IM9 to sell