I'd say, given their current form, both would be equally dangerous for any of the top players in small to medium tournaments, so it’s 50/50 for me. Nishi beat Nole just last year and Raonic beat Andy twice this year, but then again, Andy is not as consistent as the other top 3 yet, so it’s understood why he’d lose 2 matches to Raonic. Also, while Raonic can be very dangerous with his serves, Nishi can be equally dangerous with his ground game. I’ve seen Raonic not serving as well when he’s under pressure so that can also be a liability for him.
Djokovic was clearly unwell that day (no disrespect to Nishikori) and Nishi has been very ineffective in his few other encounters against the top four. Raonic on the other hand is clearly not fazed and has put up several close fights against Federer as well as beating Andy twice. So ye, I can see where the bookies are coming from.