The VS and VF both came along at the same time, so I wouldn't call the VS the original. These days the only significant difference between the VS and VF is that the VF has a built in flow meter, which allows it to be programmed to maintain a specific flow rate. Long ago the VS didn't have a timer, but it got a timer a few years ago. The XF is optimized for very high flow rates and 3" plumbing.
I recommend the VS over the VF. The flow meter, while entertaining, is not required and not worth the extra expense. I would only get the XF if you routinely require flow rates over 100 GPM.