I have a vertical foregrip on a clamshell carbine with a bipod mount. The bipod brackets ate my hand when I held it by the forend, so I added the vertical foregrip to get my hand out of there. That's the only reason - I don't like the way guns handle with vertical grips, much less stability, and way less comfortable for carry. When I take the bipod off of that rifle, I take the vert grip off too.
I have angled foregrips on a couple rifles, and I like them so far. I have some mobility issues in my wrist, so it's hard for me to commit fully under a rifle, the angled grip relaxes some of the pressure on my wrist. AN ANGLED FOREGRIP IS NOT AS STABLE AS HOLDING THE FOREND ITSELF, but it's serviceable.
Mostly, foregrips, whether vertical or angled, just look cool, and don't really make anything better. But if you think you need one, then the angled grip is the better of the two.