I did my ME/IR on a PA-34 in 1978 and then instructed on that and the PA-44 for about 6 years. Ended up with about 1000hrs on both.
Next types were: PA-23, PA-31 for which the previous experience was very useful; and Turbo-Commander 690 and Dove for which it was not!
Then came Citation and first EFIS with the 737-300/400.
I think that the basic IF scan patterns and the need to develop ones own SA ‘picture’ (as previously mentioned) have been very useful, particularly as I still fly basic SEP. If your goal is to fly the latest equipment and never go back to something older, then the DA-42 is a good choice. If you might want to continue to fly something with round dials, especially in IMC, then the PA-44 is the way to go.
I ended up on B744, Airbus and 787 and never knew what a DA-42 looked like until I tried a simulator about 5 years ago. The panel was a bit alien to start with but my excellent host soon explained the basics. I guess my basic PA-44 skills didn’t hold me back on any of my type conversion courses.
So, in summary, it depends on your desired career-path.