Which route do you reccomend for me to buy?
The Streamlines Scenic Sub, or
the 3DTrainStuff Tehachapi Pass, or
Maple Leaf Tracks' Kicking Horse Pass?
They are all good. I have them all. If you get the updates for the terrain in Tehachapi, I would recommend that one over the others. I got the scenic update which puts more realistic scenary and track on the route. Also got the SP expansion which adds some more interesting activities. There are lots of activities available for it.
Kicking Horse has a few tree upgrades available for it which helps improve the looks of it.
The locomotives and rolling stock in the Scenic Sub is first rate.
If you really want to feel like you work for a short line, I highly recommend Richard Garber's Canton Railroad. With all 4 parts, it will keep you busy with new activities for a long time. It is a little pricey (http://www.edmfamily.com/msts/co.htm) ($41 for the full Canton-Suddenly-Thompson-Grove City CD, $36 for just the download version) but you definitely get your money's worth.
Another good one for real feeling railroading on a shortline is the A&0 Sub by V scale creations (http://www.vscalecreations.com/AOtext.htm). It models the all Alco Arkansas & Missouri as if it had taken over tracks in Northern Arkansas. There is even a set of activities which are part of a new engineer orientation to the railroad, which is similar to the way the Canton route proceeds.