I like Torque's mapping system a lot more than I like Source's mapping system. Having terrain run on forever is a nice feature. It saves the hassle of having to make a 3d skybox, and it's really works well for non-linear maps. I'm pretty sure that even if you had the source code the source engine (and last time I check the price, it was "contact us for a quote," which roughly translates as "you won't be able to afford this in your life time") you'd be locked into using the same map format that limits your level size (unless you do something awesome like decrease the player size).