Therefore am I to conclude that one device is better that the other?

For a side by side, co located devices, the ragweed correlation is moderate and am surprised by your findings. I expected much higher correlations, and have seen higher in other studies.  
 

But for a population of allergy sufferers , I rhetorically ask, is this lower than expected correlation relevant anyways.
 

 If reported as high, medium, low as categories, rather than numbers, the two devices reporting would probably be very close. This is one way to standardize, currently done by the NAB method of reporting, by categories instead of numbers.