I like writing comparisons. Simply writing a review of a single item doesn't have a good chance to rank in the search engines, because there are already tons of reviews out there. There's likely to be much less competition for searches like "[x] vs [y]" or "differences between [x] and [y]" or "compare [x] and [y]."
Please, it seems to me that people comparing two specific products are probably pretty far along in the shopping process. Therefore, they're likely to click through and buy something if you provide them with the information that they need.