I'm of the opinion to always make your own opinion hahaha. Nobody knows for sure and there are so many ways of targeting that it will require a lot of promotions to figure out which settings were most successful.
What I usually did was promote one of my videos by targeting them at specific videos and/or channels that match the same of audience I'm looking for. This so that I maximize my conversion rate from viewer to subscriber. I haven't run any adwords for quite some time now so my last campaign is a few months old now. I do know that
@KiddieToysReview had better success I think with using very targeted keywords and let Google find the best audience. I tried it as well and the result was similar for me. My best advise would be to create a few small ads first and don't run before you can walk. Otherwise you'll blow all your budget with bad settings or maybe on a video that you think is great but doesn't translate into good subscriber ratios. YouTube is a marathon, so don't worry that it takes a couple of weeks before you find what works best and only then crank up the spend to what you want to invest.
One final thing, I always made it a sport to get 1 cent ads and I almost always managed. But targeting RyansToyReview proved to be the hardest channel, I never got more than a handful of ads run on their channel, the demand for ads on their channel is just too high and increases the cost for advertising. So try to find which channels work out for you, maybe some with over 100k subs as well as over 1 million subs. Than check after a day or two how well it went and which drive better results.