Ya know, that's a hard question. Both ReAction and MUI are ways to draw UIs with clickable buttons, lists, widgets, all the standard stuff, and they both do it by "drawing" the pixels of course, and since ReAction is the OS set of tools for drawing these things, then that is the OS way of doing things, but if OS4 is provided with both ReAction and MUI then MUI will also be the OS way of doing things.
MUI is pretty configurable so that you can change the look of the UI for all MUI progs you use, but so is reaction (but to a lesser extent). ReAction has fewer gadgets defined as well, but that is probably because MUI has always existed outside of the OS, and therefore people have felt freer about making new gadgets for it than they have with the OS gadget system.
ReAction builds on Boopsi, so it too is object oriented.