For the SEO Ranking which speed should be choosen cable 5 Mbps or Native
Since GG has always been focused on maximizing UX my best guess is Cable for desktop.
average “mobile 3G” for mobile website.
If you are focus on SEO ranking you may try to improve something right ?
Try to improve “speed index” 
ps : if you want to improve crawl efficiency you should monitor googlebot’s “HTML” loadtime in your access log
regards,
Thomas