I guess you need to actually test different sites to see for yourself. Personally I wouldn't buy subscribers, at least not a lot of them. High retention views and engagement is much better to rank videos. Just keep in mind that your videos needs to be engaging, if you want to be successful with ranking videos these days.
Everyone worries about subscribers, if you look at YouTube groups on Facebook groups, they all want 1,000 subscribers. I know thats part of the magic combination for monetization but what's the point in having those subscribers if they aren't watching your videos?
Focus on your content and share the hell out of yours videos, if people like them, then the rewards will follow.