If you don't mind either making a hole in the guitar or removing one of the existing controls (knob, switch, whatever), I would go for the guitar-based one, too. The only advantages of the foot-based one that I can think of are that it will apply to any instrument you play through it, and that you'll have both hands free for playing. The big drawback is, how good of control will you have with your foot, and how fast will you be able to do it?
I'd go for a switch on the instrument itself. You can get a pushbutton momentary toggle that will work perfectly. Something like
this would be my first choice. It's normally open, so if you wire one terminal to the output jack "hot" and the other to the output jack "ground", when you push the button you'll be more or less shorting the amp's input jack, which is the equivalent of unplugging your guitar cable (the amp's input jack shorts itself when there's no cable plugged in).
You can use a floor box to do the exact same thing, but then you'll need a switch that's designed to be pushed with your foot. And yeah, momentary, not latching!