OP, I vote for the HK. I have the VP9F, and I love it so much that I'm buying at least one more, plus the VP9K, and I hope to get the VP9CC, too. Eventually. I don't own the Echelon, but it is very dynamic and will allow you to change configurations.
However, what I've discovered about changing configurations, like you can with the p320 (I also have the Legion) and the p365 (I own the XL and XMacro), is that I generally only change until I find what I like, then shoot it as much as possible. If your goal is to play Lego with your handgun, then accessorize it, these firearms are very fun, with endless possibilities.
I prefer to spend my time, effort, and dollars on training and practice.
The VP9a1 is incredibly accurate and more solidly built than most other pistols, with years of testing before it was released. Surprisingly, my VP9F is more accurate in my hands than the X5 Legion right out of the box, with no modifications. Shooting groups of 4-6" at 25 yards is possible for me with the VP9, though I'm working on consistency. I never really managed that on the X5 Legion (never even within an 8" target), for some reason.
The HK barrel has a number of advantages compared to most other striker fired pistols. It is cold hammer forged, which improves durability and (according to some) accuracy/precision. It has a polygonal rifling instead of the more traditional land-and-groove rifling, which provides a better gas seal. This will increase velocity and prolong barrel life, from what I have read. I believe the chamber also has some differences, allowing for more consistent extraction/reliability as well, but I can't recall the details at the moment.
For me, I just enjoy shooting pistols. For fun at the range, almost anything ends up being fun. But for reliability, I prefer HK and also Glock. This especially comes into play if you shoot more than a few hundred rounds through the pistol every year. Or ever. at lower round counts, I think any major modern pistol people recommend will do.
I've only put about 900 rounds through the HK in the month I've had it, so I can't tell you from personal experience about long term reliability. But the accuracy has been fantastic. If you can easily carry a compact like the VP9A1K, I would choose that pistol without reservation.