[mija]

Will a rpi 3 b+ run kodi fine?
1080p, streaming content.

Yes it will run 1080p Kodi fine so long as you stay away from high bitrate 1080p 10bit HEVC/H265 material which will really strain the old RPi hardware and possibly cause overheating an throttling problems if you really push it. There is no HEVC hardware decoding on RPi's.

It why a lot of people have gone with cheap out of the box solutions like AMLogic S9xx devices running LibreELEC Kodi Krypton where everything is hardware decoded without breaking a sweat. Some devices like the MINIX ones even come with integrated quality Wireless Mini Keyboard remotes.
For non HDR - 1080p the AMLogic S905 - MINIX U1 is a nice bit of kit with it's A2 lite wireless remote.

Wireless remotes are much more responsive vs Infra Red only. Makes for a snappy LibreELEC Kodi Krypton setup.  

So if it is a case of Shield vs Rpi3 b+ you would lean towards shield? I do have a modern nvidia gaming pc too and play games and I guess the extra functionality of the shield as a game-streamer could come in handy as well, also neat with voice-controls which kodi 18 seems to support on the shield as well.