Discussing topics like this in the absence of some relative context will seldom yield any useful information. So with that in mind ....
A good leather holster (like the Double Diamond) is a great option until it gets really wet. It can take days to dry out and until it does, it will hold that moisture right next to your gun. That's obviously not a huge problem with some guns (like a Glock) so you'll have to decide if that's a relevant concern under the conditions you'll likely encounter. Obviously, wet leather is subject to mold, mildew and losing it's shape with repeated exposure.
As to the HPG Kit Bag, I have found it to be a very useful piece of gear, but not when getting the gun out in a hurry is a requirement. Now, I understand "in a hurry" is a subjective notion, but l can be reasonably confident in saying it's slow (probably too slow) compared to most other open carry options. More importantly however, presenting the gun from the Kit Bag safely and predictably requires two things; a body position that's conducive to getting the gun out and the use of both hands. In other words, the Kit Bag works best when you're standing flat-footed and using both hands to present the gun.
Hopefully, you can see the potential problems with that method of carry without any further discussion.
