For many folks it comes down to preference and features.  You can have all three installed on your computer.  Know that you will have to pay for POD Farm 2.5 and you can no longer get the reamping plug-ins for GearBox & POD Farm 1.

I will outline some advantages of each.

GearBox 3.72 - Still available for download.  Stand-alone software is free (plug in can no longer be purchased). It's the software that will work with GuitarPort Online or edit tones on POD X3 series.  Downside, you cannot create dual amp chains (Except for X3).  Limited flexibility for effects routing and arrangement

POD Farm 1.12 - Still available for download.  Stand alone software is free (plug in can no longer be purchased).  Will work with all GuitarPort, TonePort, and POD Studio devices.  Can create dual amp chains. Limited flexibility for effects routing and arrangement.

POD Farm 2.51 - Current product.  Must pay for stand-alone software (includes recording plug-in).  Will work with just about any input device, including non-Line 6 devices.  Complete flexibility of effects routing and arrangement.  There is alsl a free version with a VERY limited set of amps and effects.

I have an POD X3 live, as well as a subscription to GuitarPort Online, so I still use GearBox.  I also use POD Farm 2.51 for direct recording.  I have all three installed on my computer.

Bottom line - I would install GearBox (for familiarity) and POD Farm 1 to get a feel for dual amping.  If after testing out POD Farm 1.12, you decide you would like mroe flexibility and the VST reamping plug in, then check out POD Farm 2.51.  The amp models are the same between products, so no real sound advantage for any of them.