Chris... I don't know where you got that $400.00 price difference? But, it's simple... a slider fits in places that a dolly won't, it's easier to set-up and transport...and you can get all kinds of different shots with a slider (dolly in, dolly out, horisonal slide, vertical slider, dutch angle slide, etc...). If you need anything longer then 3'-4'...you should be looking into a dolly, not a slider.
Hope that helps...
those are digitaljuice slyder prices. my cam rig is close to the 30lb mark , i ended up purchasing the 40inch version with a high hat, C-Stand quick release clamps.


