A metal obelisk trellis typically features three or four sides. The right one for you depends on a couple of factors.
3-Sided - Since this trellis has only three sides, it means that fewer materials are used to construct the product. Hence, it has the advantage of being cheaper. If the budget is a bit tight, then it's an excellent option.
4-Sided - A four-sided trellis will typically feature results that are bolder and more imposing. Thus, if you want to maximize the "exclamation mark" effect of a trellis, then this one is a suitable choice.