Since I get this question a lot, I decided to write a short monograph on the subject.
First, frameless glass shower enclosures require a method to anchor the fixed panels of the shower to the tile. That is why you need either clamps or U-Channel. Both of these “anchoring systems” are quite suitable from a structural
standpoint. So your decision boils down to which “look” you prefer.
Having said this, there are a few things to consider:
a) if your walls are bowed, U-Channel will mask the bow in the wall whereas Clamps will tend to highlight it;
b) because of the additional fabrication costs, Clamps tend to add expense to your shower enclosure;
c) if you are re-modeling your bathroom in anticipation of selling your house, choose Clamps. When customers (particularly younger ones) express a preference in this regard, it is always in favor of Clamps.