The specs are, without any doubt, sensor dust. If you have sensor cleaning on, every time you turn the camera on and off, they will, more likely than not, start cleaning themselves. Anyways, access the sensor cleaning function a couple of times manually to force more 'shakes', and they will be OK.
The worst cases are those ultra blue skies that get the dusts clearer in view... for most pictures you won't notice them at all.
Now, as for the variances in cameras: the sensor is NOT to blame, at all... every model camera has the same sensor and variation among them is not relevant. What IS relevant is the positioning of the sensor and possible miss-alignments. Lenses suffer from this as well... so, your first camera (which probably has some problems with sensor alignment as do all cameras!) is a much better MATCH to your first lens (which alignment problems could actually offset your camera ones, making them a perfect couple! ) than your second camera is to your second lens, in which case the issues of one appear to accentuate the issues of the second.
So, say your first camera's sensor is 5 microns too forward, but your lens has the focus convergence point 5 microns forward to the zeroed specs... then your lens is perfectly focusing on the sensor plane. Imagine your second camera has the exact same variations, but one of them in the other direction: the sensor is 5 microns too backward, and the lens has the same 5 microns forward focusing... now you have 10 microns of missaligned images!!!
So, yes, for sure, keep the first camera (at the discounted price!), use it a couple of weeks, retest for dust specs (with the procedure you did). If they are the same, you need to clean the sensor (very carefully! special care with not blowing additional dust or worse grease or other substances into the sensor!!!). If they are reducing in quantity, just keep selecting the sensor cleaning option a couple of times in each shooting session and allow it to clean the camera when on or off as often as possible.
Keep particular care in not changing lenses in dusty scenarios, and don't use mirror lock up features or change lenses with the shutter open.
Hope this helps calm you down a bit , best regards,