Both do reasonable high pressure (my high volume-one does 110psi). The high volume one presses a bit more air in per stroke, which translates to less work at, say, from zero to 60-95-psi. But as the pressure rises, it is harder work up there than the "high pressure" pump. Or, to put it simply: The "High pressure" has a smaller diameter piston, and thus less volume, and the other has more internal volume.

My "high volume" does really good, even at 95-100 psi, so it's only if you have high pressure road tyres and/or are weak or petite that the high pressure one would be handy. (Edit: But it will be slower to pump up more voluminous tyres until you reach that high pressure where the other would be struggling /end of edit).

The "G" at the end of both the HVG (high volume) and "HPG" stands for "gauge". So you don't want the HV or HP, you'd want the HVG or HPG (my recommendation is the HVG).

The pump is just under a foot long and weigh next to nothing. It also comes with a holder that sits next to the bottle cage utilising the bottle cage bolts. I haven't used mine, as it is just as easy to carry it in a bag along with my other stuff with no risk of someone being tempted to nick it.