Making something float simply means you increase the item's volume (size) relative to its weight.

In this case it looks like ACR increase the volume, and it turn increased the weight a little - but obviously not enough to stop it from floating.

Floating is nice IF it doesn't float away. The only way to keep it from floating away is to attach it to something secure. If its attached, then you don't really need it to float.

... unless you are likely to be treading water, in which case this means that you don't have to hold it up while still treading water (assuming that the unit floats with both antennas are sufficiently out of the water and pointing to the sky).

I'm on land or a swimable distance from shore, so I'd go with the smaller unit. If I frequented the ocean or other large water bodies I might get the floating unit.