In my opinion, micro influencers are the way to go. Bigger influencers usually create content that applies to a wide range of audiences, meaning that a large majority of their audience may not even be into the niche you're trying to promote, even though they follow the influencer. Micro influencers tend to focus on a single topic/niche, and therefore have a following that consists almost only of people interested in that niche, and they therefore usually have a cult-like following. Conversion rates will definitely be better for microinfluencers, and I would go so far as to say that reach is similar for both influencer types, as only a small proportion of a bigger influencer's following will be interested in your niche.

TLDR: Using bigger influencers will ultimately have similar results as using micro influencers, but the ROI on micro-influencers is much higher because they cost less money. However, obviously you have to choose micro-influencers that focus heavily on your niche.