I'm LGBT and a non-URM minority and participated in SEMJF last year. I echo the sentiment above: firms generally take school OCIs more seriously, but it's not to say they won't call back anyone. I think at SEMJF, they specifically require firms to report to the job fair committee their callback rates. Anyhow, for major firms that have some connection to your school, it's probably best to bid on them through OCI. For ones that don't visit or have few interview slots, maybe allocate those to the job fairs if you need to free up bid slots for other firms. While I struck out from SEMJF (mainly due to the market I was focusing on), I did get offers from another, similar job fair.
As far as this is concerned:
I'm wondering if there is any advantage/disadvantage to interviewing with these firms at SEMJF as opposed to waiting until OCI (where I would not emphasize diversity issues)?
The only advantage is that you get to interview before OCI and get one of the first callbacks. Depending on the firm, this advantage could be pretty significant. Even at SEMJF, your interview would be no different from a regular interview—there's no need to "emphasize" diversity issues just because you're at a diversity fair.