If the following two conditions are met then flag for moderator attention using the "other" option.
- The question is a good question. Migration is not the way to get rid of bad questions from your site.
- The question is really off topic for your site. There are a number of sites that have overlaps with each other. Just because it's on topic for one site doesn't necessarily make it off topic for your site.
Additionally, it will probably help if you are an active member with a "reasonable" reputation of the site you want the question migrating to. This shows us you have an understanding of the target's site scope and audience.
If these conditions aren't met then deal with the question on the site it's currently on.