Thanks for that reply Toowheels. I agree with your point that a jacket like the Sand or Horizon will not be the best choice in very hot weather, so when I'm on a local trip in the summer I will typically bring only a mesh jacket. I found that slipping on the waterproof liner from the Sand jacket under the mesh jacket does fine for quick summer showers too.
The problem arises when on an extended trip when I only have room to take one jacket along. For example, I've made a few cross country trips where I've encountered 30 degree temperatures in the mountains and temperatures in the 90's crossing Nebraska, not to mention rain, fog and hail. The Sand jacket, along with a Gerbings liner, has done reasonably well under those circumstances. It's hotter than I want it to be on those hot days, but it's bearable. My biggest complaint about it is there isn't always a place, or time, where you can stop and put in the waterproof liner before it starts to rain. It can be tempting to just put the liner in if there is any possibility of rain, but on hot days with intermittent rain , where sometimes I want the liner and sometimes I don't, I would really like a better solution.
I'm hoping that the Horizon jacket might not be much worse than the Sand on dry hot days, when I can open the vents, but will allow a quicker transition from vented to waterproof on those days with intermittent rain. Also, I would like it if the jacket didn't become saturated in the rain. I can open the vents on the Sand even when the waterproof liner is in place, and do feel some air flow, but most of it gets blocked by the liner. The vents in the Horizon would seem to be better when opened, than open vents in the Sand when wearing a rain liner.
It is my fear, however, that the Horizon won't breathe as well as the Sand, as you have pointed out. The venting capability of the two jackets look similar though.