Buy the largest you can get.
It gets confusing because of all the marketing BS. There is no single ‘rating’ like on a fuse for instance. Roughly 20 amps, blows, less than 20, fuse works ok. On a valve, it is mostly a matter of pressure drop. Pressure drop is a function of flow squared, so 2x flow = 4x pressure drop. It increases rapidly with increasing flow. And pressure drop is heat.
So one company might rate a valve at 20 gpm at say 100 psi drop, another rate the same valve at 25 gpm but at 150 psi drop. ‘25 gpm, or 20 gpm, I will buy the bigger one’, marketing……..
At some really high flow, the pressure drop and forces can cause unbalanced forces on the spool and mechanically malfunction, but that’s way beyond discussion here.
20-30 gpm is a common range of most small valves due to spool size and manufacturing reasons. And it covers most small mobile equipment needs.
Some are offered with 1/2 NPT ports or 3/4 NPT ports. Same internal spool cuts, same casting passages, just different ports. Same pressure drops in the actual valve, but bigger ports do allow bigger adaptors, which does reduce the pressure drop.