George, I can't understand why Acurite would sell the old one panel 5 in 1? Didn't they upgrade to the 2 panel design because the 1 panel design failed to give accurate data?
Not everyone benefits from the two-panel unit. In many parts of the US, a single-panel unit can work just as well as a two-panel unit. Maybe even better. It all comes down to the angle of the sun over the course of a year.
As a general rule, the two-panel unit tends to work better as you move further south. That's because it catches the sun better in the early morning and late evening to keep the aspiration fan running longer.
As you move further north, the single-panel unit tends to work just fine. Some have found the two-panel design is worse up north, particularly around solar noon, and have reverted back to the single panel.
I've been running both single and two-panel units in Nebraska and Texas. It doesn't seem to make a noticeable difference for me.
So... Acurite give you options by offering both. You can save a few bucks if the single panel unit works fine for you.