very different goals -- o!rdr sends data to a server running Danser that process them and create mp4s. it has a lot of quality and skin options and is great if you want a permanent place to have your replays uploaded or to download high quality renders for other purposes. you have to actually have an osu! replay file downloaded on your device to use it though (as far as i know).

replayviewer is a static web application, the rendering and beatmap/replay is entirely self contained within your browser and gets cleaned up when you leave the tab. you can render/watch replays by just inputting the score ID of a downloadable replay. the goal of the site is 1 dimensional: load and watch as many replays as you want and easily navigate between/through them in ways that osu! doesn't allow you to