One key characteristic when deciding on which to use is what you want the final didj to look like as far as the shape and texture goes. This only comes into play if you plan on heating and bending/forming your didj to give more of a natural look and shape.
WARNING! The heating of these synthetics will produce toxic off gases. One should take proper precautionery measures to ensure their own safety!
I personally just try to heat on a day with a slight breeze and make sure I'm up-wind. If I were to do many more of these type didjs I would definitely invest in a proper mask system.
When heated, PVC will tend to take on the characteristics of a rubber hose. It bends like one but the surface tends to stay very smooth. ABS on the other hand when heated the surface tends to distort and blister as well as becoming very rubber hose-like. Now this at first may sound like a bad thing but there are many advantages to this effect. One of the things I like to do with ABS is to heat the surface for a very short time. Enough to make it 'sticky' on the surface but not rubbery all the way through the thickness. Then I take leaves and press them into the softened surface to make perfect impressions. It is a nice effect.