If you have a highly portable TIG power supply (which is easy to move to the workpiece being welded, but keep in mind you'll probably need to move the tank as well), the 12.5' lead length will probably be optimal. Excessive lead lengths aren't necessary desirable. Extra electrical resistance, possible harder HF starts, extra EMF emissions in AC mode or when HF starting (from a big coil of leads) etc. It's not just about cost. Its also harder to move the extra leads around (and it's not just about torch lead length, but eather, the footpedal, and ground all need to be lengthened so the system is effective).

If you have a portable setup, it's best to try to keep it portable. 25' leads are probably not the hot ticket for maintaining a portable setup. To maintain a highly portable setup that you can still move around far away from a plug, consider lengthening the power supply cable instead of the TIG leads. You don't necessarily need thick, high-gauge conductors to accomplish this. I believe I needed only 12 gauge conductors to extend the power supply cable of my 200 amp @ 60% duty cycle machine an extra 25'.

If you don't have a portable machine (and generally speaking, a watercooled setup is not generally portable, unless you have it on wheels) you'll probably want the longer 25' version, unless you expect always being able to bring your work close to the machine (which depending on what you're welding, may not be practical).