I'd vote for the PA44. With the Diamond's single power lever FADEC engine control, your initial multi-training would miss out on the critical "mixtures-props-throttles" routine during (simulated) engine failure. The FADEC greatly simplifies this step, and, yes, that's how jets do it, but, you're most likely to start out your career in a piston twin with six engine control knobs, not just two (Baron, Seneca, twin Cessna, Navajo).
That said, ultimately, you'd have required initial/transition training for any new make and model, so it might not matter that much in the long run. Other factors such as overall cost, convenience and proximity to your home, scheduling, etc. probably weigh more in the decision. If you're talking FAA, there is no restriction between the DA42 vs. Piper that I know of (unlike a center-line thrust only limitation if one learns multi- in a Cessna Skymaster, or some military fighter jets).