Craig, I may be wrong in the following, if so hopefully others more knowledgeable will correct.
The audio, guitar direct if an electro-acoustic/electric and vocal via the mic, is routed into the PC using an audio interface. The computer will use appropriate drivers to handle the audio interface.
Given that the quality of the recorded audio in Reaper and OBS will be the same. At least I think so as I’d assume this is driven by the interface and drivers.
In Reaper you can do processing on the recorded audio after completing the recording. Can be as simple as balancing levels and panning, or more sophisticated using fx plugins, eg to add reverb and delay, EQ, compression, etc. Once satisfied then you can render a final audio file.
OBS is simply a recording (or streaming) application. You can source multiple inputs separately from the interface using the same drivers. But the final video saved will have the combined audio, no opportunity edit individual tracks after recording.
That said, you can setup levels to balance vocal and guitar before recording. And OBS enables the adding of fx on each track, which it calls filters. If you have Reaper setup then OBS will be able to access the plugins as filters. You can also work with panning in OBS. The downside is you need to set everything up before recording. Initially I found that quite time consuming but once you get the hang of it, it is pretty quick.
As Toby said, you can record audio in Reaper and OBS concurrently. Then you can use a video editor to import the video with audio from OBS and a process audio from Reaper. Many video editors have automatic syncing that will align the video with audio and the audio-only source. Actually while Reaper is a DAW, it has some basic video editing capability, and I have produced a video with me playing 4 parts in Reaper, syncing up all the video and audio.
So I see no reason why you’d not get similar quality from OBS as Reaper, with the difference being the effort is upfront with OBS to set it up and may be after recording with Reaper. These days I make videos for AVOYP and stream to Zoom for OMs just using Reaper. I find the fewer apps in the mix the simpler.
Hope that makes sense, ask if not.