The number one question is about your wardrobe. Can you wear a fairly large cover garment all the time? If so, you can carry in an OWB holster and that really opens up your carry choices. The bulk of the gun isn't an issue. If you think you need an IWB holster, the HK pistols are really friggin' fat and there are far better choices if this will be your mode of carry.
I think a lot of people choose IWB holsters without really thinking about it though. With IWB, you still need a cover garment because the butt of the gun is visible above your pants. If you have a cover garment, why not use a OWB holster? If your over shirt is untucked, but not terribly long, I can see the benefit of IWB, but if you select your shirts correctly, you can do OWB in most cases.
BTW... I do carry an HK UPS .45F on occasion. It rides in a Blade-Tech OWB holster. I usually carry this when I can get away wearing a 5.11 vest in public.