I am a twin cylinder fan. I have owned a triple 600 cc and aways felt it lacked the power off the start. Generally the twins generate more tourque off the start and as lighter weight. I recommend the twins.
EFI is the way to go, I agree. I love being able to get started at any temperature in two (or three) pulls. Plus you never have to change jets or worry about carburators. The bad think about EFI, for me anyway, is that if there is a problem I have no clue how to repair an EFI system.
Three cylinder sleds have been almost entirely phased out in the snowmobile industry at this point. The twins cylinders are offering the better performance and reliability (1/3 less parts).