Having used both, S3D offers much faster slicing and a simpler user experience, with a library of presets for most common printers in terms of printing. Choose the correct preset, import the model and hit the slice button.

Yes, you can change more than you can in Repetitor, but it's tweaks rather than anything more. Infill and supports are better in terms of design and printing and the finished items do print to a better quality.

S3D can also repair errors in the STL file, but as you can do this for free in Microsoft 3D Builder, it's not much of a gain.

Worth the money? Well that's a moot point. It depends how much printing you intend to do, as there is nothing particularly wrong with Repetitor, but S3D is smoother and a little quicker.