Okay so I did a little experiment - I reinstalled shadowplay to do the testing to see which gives you the best quality with the same amount upload rate. I chose 9mb for both because that's what shadowplay set me to by default. I picked 1920x1080p and HD the highest settings for 1080p I could select. I'm running both shadowplay and obs at 9mb upload rate.
Now I set OBS bitrate the same as shadowplay 9000kb/9mb but obs has a little better control over your video quality than shadowplay. I gave the quality one tick up in obs to "Indistinguishable Quality, large file size" imo is a low impact nice improvement as you can see in the video's I'll post below. Open them side by side and give them a watch, with out question OBS has better quality and smoother playback. Shadowplay quality is less (more pixelated) and stutters much more than obs.
For the time being I'm going to stay with obs, it works best for me.
OBS 1920x1080p 60fps 9mb Up
Shadowplay 1920x1080p 60fps 9mb Up
I just found these, links below, I will run a few more tests tomorrow.
Stream Settings Estimator
Live encoder settings, bitrates, and resolutions
Recommended upload encoding settings
EDIT - 2018/12/18
I probably should have mentioned I am on a 1GB fiber line.
Okay so I did a couple more stream tests, this time I set shadowplay to custom 15mb and I did the same for obs 15000kb and reran them both. The results are linked below. Both video look worse imo obs is more clear but more choppy while shadowplay just plain looks terrible. You be the judge.
OBS 1920x1080p 60fps 15mb Up
Shadowplay 1920x1080p 60fps 15mb Up
While OBS does take a little time to get set right and tweaked in, imo it is clearly the better of the two with a clearer quality stream with the same upload rates. If you're looking to stream up some video and quality isn't a top priority then Shadowplay is a great choice for you, it's super easy to setup and easy to use while obs has a few settings that need to be dialed in.
I hope this helped someone out and please feel free to add your thoughts and comments so that we can discuss them.
EDIT - 2018/12/19
Ok I know I saw dogs post about "shadowplay settings", he must have removed it.
Okay, I went ahead and ran a few more stream test last night. I did as dog suggest setting shadowplay at it's highest settings 1440p 60fps and slid the slider all the way right to 18mb. links below
I ran obs with the same settings as SP 1440p 60fps 18mb up but my video biterate was a bit off and made the video choppy. So, I bumped up the VBR to 25mb and re-ran it, link below. OBS has two main settings you need to dial in properly, Biterate and Video Biterate. If either one is off the stream goes choppy. I've got the info on the video biterate but I can't find anything on the biterate. I ran it at the recommended setting according to the Stream Settings Estimator
3500 but that seems to low because I get stuttering when I run vbr at 18mb so I bumped br up to 5000. I'm going to run a few more test today see if I can get my stream smooth as possible. I think I am very close to having OBS dialed in.
Shadowplay 1440p 60fps 18mb Up
OBS 1440p 60fps 25mb Up
Okay - Did a bit more testing and came up with the following settings based on the default settings. VBR is optional 18mb is low as I went, max based on my upload speed I found 30mb is about my limit. I'll stand by my original conclusion that for the same amount of upload speed obs puts out better quality.
OBS 1440p 18mb
My OBS settings
Soon as I figure how to upload and attach my images will get them posted.