Should you build runic echoes or cinderhulk on gragas

It depends on matchups and elo. Echoes is better in lower elos and versus squishy targets but you have to know your limits with this item. Cinderhulk is good if you want to go tank and can rely on that your allies do enough damage.

I self prefer Cinderhulk and full tank build since it provides a lot more secure game and in fact you are still able to kill carries.

Quick tip: Build cinderhulk when the enemy jungler does the same.

.......I just like my tank jungles....kind of my preference butt yes if your team needs a tank build tank....if not build AP and crank out some heavy damage.

The reason why people build Runic Echoes is because of the extra movement speed / damage. It's enough to carry you through the first 20m of the game. Build the item if you see a win before 30m, and you need to build hybrid offensive / defense items, like Gauntlet / Abyssal. This way your damage stays relevant, and they still can't kill you.

Build Cinderhulk in AP heavy compositions where your damage won't be relevant, unless you build full damage. Build it when your goal is to hover on your carries, and keep them alive. Personally, I prefer the Cinderhulk build, because you just never die.