The only real function difference between the SM-EW90A/B (under stem) and EW-RS910 (bar end/frame) is the number of ports, 3 or 5 for the SM-EW90 and 2 for EW-RS910, which only matters if you're adding accessories like sprint shifters or the climbing shifter...and don't have open ports on your levers.
Like others have mentioned, the EW-RS910 requires bars that have holes to route the wires, so you're constrained by that for upgrades/different sizes. Some manufacturers will use the EW-RS910 and route the wires through both the bars and stem, which means having to find both components that are compatible. None of it's a big deal really, just preference on what it looks like, supporting components, and how easy/difficult it is to install.