To me, striker fired vs traditional DA/SA is the whole question. The action and function of the pistol is the most important issue. You can get a top quality pistol with either type of action, and both the HK pistols are fine choices for quality and reliability.
With that said, I am a strong advocate for striker fired. The primary reason for DA/SA is just that people who like it that way. There are valid reasons why some people prefer traditional DA/SA, but those are specific and individual reasons. For almost everyone asking questions about pistols, DA/SA is the wrong choice.
Years ago I watched a guy who had won a state IDPA championship with a Sig P series pistol. He was truly fast and accurate for the first two shots from the holster with a DA/SA pistol. But he is the only person I have seen who could really run that type of action. Everyone else who has told me they are "fast and accurate" with those pistols does not understand one of those words. Their double tap from the holster is either slow, or one shot is wild.
Get the striker fired pistol unless there is something that you just love about DA/SA, and you love it enough to choose a less efficient system. And yes, the VP9 is a top choice for a striker fired pistol.