I'm interested in knowing the difference between these two models. I mean the models, not the prototypes. I am familiar with kato engines, but have never run an intermountain. Are they comparable performance wise? Is the -2models longer or have a longer wheelbase? I have tight curves. Any other differences such as lights, other qualities, etc?
The Intermountain model is going to be similar to an Atlas SD50, SD60, or SD60M, if you're familiar with those. The drive train parts (trucks, gears, drive shaft, motor) are all the same or nearly the same. Intermountain models in my experience usually have decent golden-white LEDS for lighting. The SD45-2 has a longer wheelbase than the SD45, being (I believe) the same as an SD40-2. The SD45 is the same as an SD40. The Kato will be a better puller, especially if you get an old pre-DCC version. (But those mostly came with crappy yellow LEDs.)
Pretty much what Ben said on the dimensions. My Intermountain unit has a slower speed motor very similar in performance to an Atlas slow scale motor and runs pretty well with them for me. The Kato unit's speed will be faster. I've found that my Atlas 6 axle locos pulls about as much as Kato DCC friendly units do give or take a car or 2. My older non DCC ready Kato as well as my Atlas/Kato(non DCC friendly as well) units do pull more though. I am happy with both brands. Unless your running DCC and speed match I would hesitate to run a Kato and the Intermountain unit together due to the speed differences.
The SD45-2 and Tunnel motors share the same mech, IIRC. I have 11 Tunnel Motors and 1 SD45-2, they run pretty much alike.