Well the more reviews/products you put on a single site, the more authoritative your website becomes. If you're reviewing multiple products in the same niche/category, stick with one site. Although it can't hurt to have multiple websites, it's still a good idea to have a central 'hub' for everything, and you can link to and from that central hub as well.

You'll probably get mixed answers, and it can go either way. One side of the glass says, one site for each specific review might be more SEO friendly because its more relevant, but on the other side, one larger site is more authoritative with more content and that also could be more SEO friendly.

Additionally though, having multiple reviews on a single site, could lead to commissions on additional products, your traffic might jump from one review to another and end up buying something entirely different.

In the end, you're good to go either way, the decision is really up to you, both have their pros and cons. Being an SEO expert myself, I've had success with both routes.