If you're actually line of sight, and assuming the same antenna, the increase in range would be fairly small. Generally the rule of thumb with 70cm UHF radio is that once you go over a fairly small power, say maybe a watt or so, if you can see it you can talk to it. Where the higher power would make a difference would be in over the horizon comms, where it would extend the range to a degree.

The amount of extra range would be heavily dependent on topography, interference, how high off the ground one or other end was, and the like, so it's difficult to quantify.

If it helps, I have managed (easily) a range of over 12 miles between two 500mW UHF handhelds where both of them were in optical line of site, where the range quoted in the documentation claimed "Up to 2 miles!". On the other hand, put a large hill between them, and you get maybe two hundred yards.

70cm frequencies propagate extremely well through air, not so well through planets...

A lot of the difference between such things is the antenna. If it's crap so will the range be. The choice of which one is best probably comes down to your usage requirements and the price.

pca