Export to 60p for preservation
You will preserve all the frames and the look of your 60p sources, and the 30p frames will be doubled, but with a decent codec this will likely not affect the size of the file because the encoder will use a "frame-repeat" flag.
Export to 30p for uniform look
60p video looks "live", while 30p video looks more "film-like". Combining the two in one video may look jarring to your viewers. Then again, some viewers do not notice differences in frame rates.
Export to 30p and use 60p for slo-mo
You can export a 30p project and create true slo-mo from your 60p content, just navigate to clips' properties and set their speed to 0.5 -- no frames lost, and no fake frames added.
As for whether your viewers will be able to watch 60p - yes, they will. Any TV set supports 720p60 and most do support 1080p60. YouTube supports 60p for several years now. If you want to be extra safe, when exporting to 60p use 720 resolution, that is, export to 720p60.