It seems to be largely dependent on the heater itself...
Below is a chart created from data gathered by OMNI Environmental Services two years ago. It is from a report of a very comprehensive real-world (they used cordwood in a manner that ordinary users supposedly burned) study that compared two EPA certified wood heaters (sorry, it didn't say which ones), one a higher emissions heater than the other. Seems that heaters rated to produce the lowest total PM emissions produce less PM with dry fir than with dry maple, while the less efficient heater made up to twice as much PM with the softwood.
Results for PM are in g/kg fuel burned, not in the usually quoted g/hr. This is a much more realistic figure for us since what we should really be concerned with how much creosote we are making per cord burned, not per hour burned (which might include results for burn rates that are way lower than any of us burn in real life). Both softwood and hardwood used in the tests (16 burns altogether at varying burn rates) were at ~20% MC dry-basis (as might be determined by use of a resistance moisture meter). That is wood that is at the lowest allowable MC for the EPA testing protocol.
Earlier studies (University of Tennessee) I've seen on pre-EPA heaters ("smoke dragons") show a much worse outcome, with beetle-killed dry pine producing much more measured creosote in the flue than when burning unseasoned hardwood.
My take on this is that it all depends on how well a given stove handles secondary combustion. Most pre-EPA airtight stoves had no ability at all to handle smoke that was not consumed inside the primary burn zone. Woods that outgassed the fastest (dry pine) therefore made the most excess smoke and deposited the most creosote inside the flue. Ironically, green pine (the slowest wood to outgas) made the least smoke and the least creosote in the flue.
With an EPA certified heater, the ability to achieve much higher smoke combustion rates allows them to safely burn wood that outgasses smoke rapidly as long as the burn rates are high. The best stoves (lowest PM emissions) will handle the driest and fastest burning wood the best and produce the least creosote. The old airtight stoves did quite poorly with dry pine, which may be where the "myth" actually began in the first place.
This is quite a report that covers way more than PM emissions for hardwood and softwood, but I am unable to directly link to the PDF with a hyperlink. Here is a direct link to all of the available OMNI publications (51 in total). This one is called "Verification of Emission Factors U.S. EPA Certified Wood Heaters (Volume I)". It is the sixth publication down on the list.
http://www.omni-environmental.com/p...blications=0&totalRows_rsPublications=51&q=po
Pauls Tiegs, the founder of OMNI Labs, is currently one of the leading researchers in the field, and OMNI was the very first lab in the country certified to do the EPA testing procedure. There is much knowledge to be gained by reading these publications. Mind you, they are very technical in nature and are slow going, but everyone can get something useful out of them if they put in a little time.